With the term being thrown around left and right, I thought it would be helpful to try and attach some collective meaning to it. After all, how can two people rationally accept something is or is not balanced when they are both using different definitions?
It is my opinion that "balance" is a concept with several sub-parts and I will elaborate on them here.
The first is the one that most high-level players seems to gravitate to, both in FA, Starcraft 2, Generals and pretty much every skilled player of every strategy game ever made. It's inter-faction balance. A unit or faction can be said to be balanced if other factions (and the units therein) can reasonably counter or defeat the first unit or faction. Inter-faction balance is important such that the community can enjoy the complete variety of factions in games... that is, that no factions is substantially weaker (as not to be used) or stronger (as to be the only faction used) than the others. An example of poor faction balance is Seraphim in 3599: The faction is incomplete, buggy, and difficult to use. Another example is Cybran is Supreme Commander 2: The faction is very weak in 1v1 games and, therefore, 1v1 games are usually limited to Aeon and UEF.
The second kind of balance is inter-tech balance. All strategy games have some kind of tech system. In Starcraft 2, it's based upon tech structures, in Supcom 2 is uses Research and in FA it uses factory/ACU upgrades. A level of tech can be said to be balanced if it is neither materially stronger or weaker than other tech levels. Inter-tech balance is important such that the community can enjoy the complete variety of technology levels available in the game. An example of tech imbalance in 3599 is that Tech level 1 and tech level 4 were so overpowered that it was common to see games where neither tech 2 nor tech 3 were utilized. The original Supreme Commander suffers a different problem, wherein Tech 2 is so powerful that the other techs are never fully utilized, causing variety to suffer.
The third kind of balance is inter-unit balance. Different units within the same faction and tech level can be imbalanced if one units is appreciably stronger or weaker than other units in that same faction and tech. Inter-unit balance is important such that the community can enjoy the complete variety of units available in the game. Two examples of a unit imbalance in 3610 is the Strategic Missile Sub and the Aircraft Carrier: These units, as compared to other ships available at tech 3, can perform no valuable function well or cost-effectively. Strategic Missile Subs and Carriers are imbalanced. Another example is the Magnetron is Supreme Commander 2: This unit, far more than any other high-tech Cybran unit, is strong against enemy land units to the point that it is commonly the only unit used for this purpose, despite the fact that Cybran late-game tech has many different units that can attack land.
With these three types of balance in mind, it should be noted that a claim of "imbalance" may be justified if a unit, tactic, strategy or faction is only found to be imbalanced in one of these three ways.