What is Bargaining Power?

Legal Definition
Bargaining power is the relative ability of parties in a situation to exert influence over each other. If both parties are on an equal footing in a debate, then they will have equal bargaining power, such as in a perfectly competitive market, or between an evenly matched monopoly and monopsony.

There are a number of fields where the concept of bargaining power has proven crucial to coherent analysis: game theory, labour economics, collective bargaining arrangements, diplomatic negotiations, settlement of litigation, the price of insurance, and any negotiation in general.
-- Wikipedia
Legal Definition
The ability to overpower another in negotiations. The dominated party usually has less of something making them weaker.