It seems like Central Asia has a very nebulous definition. The most common I've seen is "everything between the Caspian and China, Russia and Pakistan", or "all the -stan countries". But just looking at the flaired topics this week, people sometimes also include Mongolia, Persia, and even Khazaria. So what is Central Asia? What defines the area as a distinct area? And what territories does it actually cover?
I would think that much like other geographic terms, Central Asia is what you want it to be, within reason. It depends on the context used, either geopolitically or historically. From my own personal standpoint, that of someone who researches the Soviet Union, I'm more inclined to limit it to the five post-Soviet states there (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan), though I certainly can see why places like Afghanistan, Iran, and Mongolia are included. So to really answer the question, one must consider how they are framing it, as that really will define what Central Asia is.