From Liquipedia Starcraft 2 Wiki
Game Balance is a concept in game design describing the fairness and balance of power in a game between multiple strategic options.
 Game Balance In General
- Overpowered (often abbreviated to OP) - a unit or ability that is the best choice in a disproportionate number of situations (marginalising other choices) and/or excessively hard to counter by the opponent compared to the effort required to use it. For example, the 1 supply/2 armor/15hp regen Roach from early Beta was overpowered in many ZvT and ZvP scenarios.
- Gimps - a unit or ability that isn't viable to use in most situations. E.g. many considered the Scout from StarCraft 1 to be a gimp. This term is not frequently used in the SC community.
- Nerf - Making a strategy or unit weaker. For example, the Roach was nerfed when its supply count was increased from 1 to 2 in patch 12.
- Buff - Making a strategy or unit stronger. For example, the Roach was buffed when its range was increased by 1 in patch 1.1.2.
 Balance vs. Fairness
- Myth: If a player wins 50% of the time when playing against an equally skilled player, the game is balanced.
- Gimps and OP units don't necessarily affect the probability of victory. Consider a game similar to rock-paper-scissors that has one more option "pinky" which always loses when used. The game rock-paper-scissors-pinky is imbalanced because it has a "gimp". No player would ever use the "pinky". Rock-paper-scissors-pinky is a fair game but its units aren't well balanced.
- Myth: In an imbalanced game with players of exactly equal skill - one player will win more often when they choose the strategic path that is overpowered.
- For example take rock-paper-scissors-wand where wand beats rock and paper. Paper becomes a gimp because wand is superior in every scenario. Wand and scissors are OP (they win in 2 situations instead of 1). The resulting game is just rock-wand-scissors. Using wand as opposed to rock (which isn't OP) doesn't improve the chances of winning because it can be just as easily countered (by scissors).
- Myth: In a perfectly balanced game, players will make their strategic choices based on factors such as their personal preferences, playing strengths, or playing styles rather than on an inherent advantage in one option.
- In any strategic game, all choices should focus on strategic viability. If more than one viable option exists, only then should preference or style weigh in.
 Implications of Imbalance
In competitive gaming, overpowered strategies quickly become prevalent once discovered. Although the game may still be fair, it would become predictable and monotonous. This may leave a portion of the game content unplayed and therefore a waste of time for the players and developers as well.
 Challenges of Balancing
The evolution of strategy in a game over time makes it difficult to determine which strategic paths are inherently more powerful due to design flaws. Players may perform strategies that are considered unfairly powerful until other strategies are discovered that counter them or are considered even more powerful. Due to this fact, some people believe inherent game imbalances do not exist because they believe new strategies will always be discovered that provide counters or are more powerful than the existing ones. On the other hand, people may constantly think inherent game imbalances do exist because they do not believe other strategies will be discovered that provide counters or are more powerful than the current ones.
Some people believe that, regardless of design, discovering and utilizing the more powerful strategic paths are part of the skill in mastering a game. Therefore, they believe imbalances cannot exist because players who use strategies that are weaker, regardless of the reason, would be considered less skilled. Conversely, players who use the inherently stronger strategic paths, what people may believe to be the imbalanced strategies, would be considered the highly skilled players.
Some people believe imbalances cannot be determined, let alone fixed, until every single strategy in a game has been discovered and tested.
 Balance in StarCraft 2
 Parameters pertaining to balance in StarCraft II
- A race or strategy may be favored in a certain match-up on certain maps. For example maps with choke points favor ranged units and maps with cliffs near mineral lines favor drops.
- A spawning position may be favored over another. E.g. the map Scrap Station has different distances between the ramp and the spawn position. This can be critical in ZvZ early game as analyzed in this TL thread.
- Units and abilities - certain units and abilities may be gimps or OP.
- Tech trees - certain tech paths can ease transitions from one strategy to another.
Any change in one of these parameters may affect others. This is part of the complexity of balancing StarCraft II.