Is it possible for more than two particles to be entangled in a quantum way? So I know that two particles can be entangled in a quantum way, but is it possible that more than two particles be entangled in a quantum way? Most descriptions provide with two-particles cases, so I wonder. (It's hard to think of three particles entangled in spin, or so.)
 A: Yes and the highest record is 3000 quantum entangled particles https://www.livescience.com/50280-record-3000-atoms-entangled.html
A: Yes, you can have as many entangled particles as you want. It might be rather cumbersome to achieve it but it can in principle be done. Multipartite entangled states actually lie at heart of a special type of quantum computation, called measurement-based quantum computation. Here, you start from a large entangled state of many parties (usually called cluster state) and by performing certain measurements on certain parties of the state achieve required state of the rest of the system. You might want to google it up, there is quite a lot of literature on this topic.
The multipartite entangled states, however have to major drawbacks - as I already said, they are not always easy to prepare, and secondly, it quickly becomes difficult to classify their entanglement. Let me illustrate this on a system of two and three qubits.
With two qubits, it is easy to decide whether a given system is entangled or not - the positivity of the partial trace is a necessary and sufficient condition for separability. But with three qubits (let's denote them by A, B and C) things start to get a little messy. You can consider three bipartitions of the whole system, A|BC, B|CA, C|AB, and look at their separability properties. Now, it may happen that the state will be separable with respect to the A|BC partitioning but not to the C|AB partition. (I am not completely sure about this, but this is the way it works for continuous-variable Gaussian states). You might even find that all three partial traces are positive but you won't be able to find a separable state of all three systems (such states are called bound entangled). So in principle, you can have states completely inseparable, separable with respect to one or two bipartitions, states separable with respect to all three bipartitions but not completely separable, and fully separable states.
And now, imagine going to four qubits. Now you can separate the system in 2+2 or 1+3 subsystems and the possibilities grow. So it becomes almost impossible to classify the entanglement of the given state. And entanglement quantification of such complex systems is even more problematic.
A: I want to add this answer, since the above question is pointed out as a duplicate for a recent one. My answer is , as I have stated in a comment above, that all quantum mechanical states are entangled, and since in theoretical principle, a single wave function can describe the universe, the answer to the title should be automatically yes.
Reading these answers I realize that the general physicist and the quantum-computation-aligned physicist give a different physics definition for entanglement. They mean in quantum computers, if in the laboratory one could control a sample so that the desired quantum numbers are entangled, in effect "know the entanglement" by construction. This is the way the question is answered and accepted, so I suppose the question was from a quantum computing framework.
In a general definition of the word entangle  in the webster dictionry though 
a: to wrap or twist together : INTERWEAVE
this is the closest to the mathematicl condition.
Entangle as it is used in physics should have new  entries.
All variables in a quantum mechanical function are entangled, i.e. the value of one depends through the differential equations on the value of the others and are consistent with the above dictionary definition. A new entry should be given  for quantum computing.
