In order for a process to be regarded as reversible, at a minimum, the system upon which the process is imposed must pass through a continuous sequence of thermodynamic equilibrium states. This is the simplest way of judging whether a process is reversible or not. But why would such a process be considered reversible? Well, passing through a continuous sequence of thermodynamic equilibrium states is only what qualifies the process to be reversible. Such a process is referred to by Moran et al (Fundamentals of Engineering Thermodynamics) as "internally reversible."

But, for total reversibility, the surroundings must also pass through a corresponding/matching set of thermodynamic equilibrium states. If this condition is satisfied, then it is possible to return both the system and its surroundings to their original thermodynamic equilibrium states, without more than negligibly affecting the state of anything else. This is the really stringent requirement for total reversibility.

Note that it is fully possible for a system to pass through a continuous sequence of thermodynamic equilibrium states, without its surroundings undergoing a corresponding/matching set of thermodynamic equilibrium states. Such a process would be considered (internally) reversible in terms of the system, but not for its surroundings, and the overall process would not be considered totally reversible. An example of this would be if you manually caused a gas to adiabatically expand or to contract quasi-statically. The process would be considered (internally) reversible for the gas, but not for your body. Your body (which represents the surroundings) experiences many irreversible interconversions of energy in its muscles which prevent it from passing through a continuous sequence of thermodynamic equilibrium states. So the system itself could be returned to its original thermodynamic equilibrium state, but not your body. However, there are other ways of structuring the surroundings for this example such that the surroundings also experiences a corresponding/matching sequence of thermodynamic equilibrium states.

So, in summary, a good definition of a reversible process for a system (neglecting what is happening in the surroundings) is that the system passes through a continuous sequence of thermodynamic equilibrium states (internally reversible process).

In order for a process to be regarded as reversible, at a minimum, the system upon which the process is imposed must pass through a continuous sequence of thermodynamic equilibrium states. This is the simplest way of judging whether a process is reversible or not. But why would such a process be considered reversible? Well, passing through a continuous sequence of thermodynamic equilibrium states is just the minimum requirement for the process to be reversible. Such a process is referred to by Moran et al (Fundamentals of Engineering Thermodynamics) as "internally reversible."

But, for total reversibility, the surroundings must also pass through a corresponding/matching set of thermodynamic equilibrium states. If this condition is satisfied, then it is possible to return both the system and its surroundings to their original thermodynamic equilibrium states, without more than negligibly affecting the state of anything else. This is the really stringent requirement for total reversibility.

Note that it is fully possible for a system to pass through a continuous sequence of thermodynamic equilibrium states, without its surroundings undergoing a corresponding/matching set of thermodynamic equilibrium states. Such a process would be considered (internally) reversible in terms of the system, but not for its surroundings, and the overall process would not be considered totally reversible. An example of this would be if you manually caused a gas to adiabatically expand or to contract quasi-statically. The process would be considered (internally) reversible for the gas, but not for your body. Your body (which represents the surroundings) experiences many irreversible interconversions of energy in its muscles which prevent it from passing through a continuous sequence of thermodynamic equilibrium states. So the system itself could be returned to its original thermodynamic equilibrium state, but not your body. However, there are other ways of structuring the surroundings for this example such that the surroundings also experiences a corresponding/matching sequence of thermodynamic equilibrium states.

So, in summary, a good definition of a reversible process for a system (neglecting what is happening in the surroundings) is that the system passes through a continuous sequence of thermodynamic equilibrium states (internally reversible process).

In order for a process to be regarded as reversible, at a minimum, the system upon which the process is imposed must pass through a continuous sequence of thermodynamic equilibrium states. This is the simplest way of judging whether a process is reversible or not. But why would such a process be considered reversible? Well, passing through a continuous sequence of thermodynamic equilibrium states is just what qualifies the process to be reversible. Such a process is referred to by Moran et al (Fundamentals of Engineering Thermodynamics) as "internally reversible."

But, for total reversibility, the surroundings must also pass through a corresponding/matching set of thermodynamic equilibrium states. If this condition is satisfied, then it is possible to return both the system and its surroundings to their original thermodynamic equilibrium states, without more than negligibly affecting the state of anything else. This is the really stringent requirement for total reversibility.

Note that it is fully possible for a system to pass through a continuous sequence of thermodynamic equilibrium states, without its surroundings undergoing a corresponding/matching set of thermodynamic equilibrium states. Such a process would be considered (internally) reversible in terms of the system, but not for its surroundings, and the overall process would not be considered totally reversible. An example of this would be if you manually caused a gas to adiabatically expand or to contract quasi-statically. The process would be considered (internally) reversible for the gas, but not for your body. Your body (which represents the surroundings) experiences many irreversible interconversions of energy in its muscles which prevent it from passing through a continuous sequence of thermodynamic equilibrium states. So the system itself could be returned to its original thermodynamic equilibrium state, but not your body. However, there are other ways of structuring the surroundings for this example such that the surroundings also experiences a corresponding/matching sequence of thermodynamic equilibrium states.

So, in summary, a good definition of a reversible process for a system (neglecting what is happening in the surroundings) is that the system passes through a continuous sequence of thermodynamic equilibrium states (internally reversible process).

In order for a process to be regarded as reversible, at a minimum, the system upon which the process is imposed must pass through a continuous sequence of thermodynamic equilibrium states. This is the simplest way of judging whether a process is reversible or not. But why would such a process be considered reversible? Well, passing through a continuous sequence of thermodynamic equilibrium states is only what qualifies the process to be reversible. Such a process is referred to by Moran et al (Fundamentals of Engineering Thermodynamics) as "internally reversible."

But, for total reversibility, the surroundings must also pass through a corresponding/matching set of thermodynamic equilibrium states. If this condition is satisfied, then it is possible to return both the system and its surroundings to their original thermodynamic equilibrium states, without more than negligibly affecting the state of anything else. This is the really stringent requirement for total reversibility.

Note that it is fully possible for a system to pass through a continuous sequence of thermodynamic equilibrium states, without its surroundings undergoing a corresponding/matching set of thermodynamic equilibrium states. Such a process would be considered (internally) reversible in terms of the system, but not for its surroundings, and the overall process would not be considered totally reversible. An example of this would be if you manually caused a gas to adiabatically expand or to contract quasi-statically. The process would be considered (internally) reversible for the gas, but not for your body. Your body (which represents the surroundings) experiences many irreversible interconversions of energy in its muscles which prevent it from passing through a continuous sequence of thermodynamic equilibrium states. So the system itself could be returned to its original thermodynamic equilibrium state, but not your body. However, there are other ways of structuring the surroundings for this example such that the surroundings also experiences a corresponding/matching sequence of thermodynamic equilibrium states.

So, in summary, a good definition of a reversible process for a system (neglecting what is happening in the surroundings) is that the system passes through a continuous sequence of thermodynamic equilibrium states (internally reversible process).