# What's the deal with Relativistic Mass? [duplicate]

So I have taken a first course in Special Relativity and the Relativistic Mass is defined as: $$m = \frac{m_o}{\sqrt{1 - v^2/c^2}}.$$

Now, when this was introduced in the course, it was introduced as something profound and very important. Not only did it show why no particle can be accelerated to a speed equal or larger than c but it also was required for momentum to be conserved in SR.

So why is it then that I keep reading here on SE that this term is deprecated, obsolete, etc. ?

This also eventually led to the famous $E = mc^2$ so I gotta ask, does mass increase in a moving frame or does it not?

This is quite confusing!

is wrong. There is no relativistic mass. The mass is $m_0$ according to what its meaning is (namely, as coefficient appearing in the Lagrangian in correspondence of some terms) and does not change; rather, what changes is the way it enters the equations of motion, which in turn show an additional multiplicative coefficient.
The terminology comes from the fact that if you want to keep and maintain the equations of motion in the same form they are for non-relativistic classical mechanics you might interpret this new term all together as a new mass, but that leads most of the times to unfortunate conclusions also in other contexts. Moreover, the bare physical mass that you measure in laboratories will always be $m_0$.