Balance of Power
defines the relative strength of a political actor
against its rivals. The term can also be used to denote the situation when the weight of a smaller actor can sway the outcome of a conflict between two large and roughly evenly matched actors. With the balance of power, the smaller actor is in the advantageous situation of forcing concessions
from the larger actors who compete for its alliegence, assuming they feel they individually are more likely to do business with the small actor rather than with themselves.
For example, in a parliament a large left wing party and a large right wing may be forced to do deals with a smaller centralist party, like passing legislation with amendments to suit what the centralists want.