There have been many attempts to define power. I will look at two of the more important ones of recent times.
The first of these was by Bachrach and Baratz in the 1960s. They posited that there are two faces of power as it is exercised by the government. First, there is overt power. This is the power to make decisions, such as passing laws. Second, however, there is a covert aspect to power. This is the power to prevent decisions from being made. A government can, perhaps, "bury" an issue so that it never even comes up for a vote. This, too, is power.
In the 1970s, Steven Lukes came up with the idea of a third face of power that was related to Marxist ideas of hegemony. Lukes called this "ideological power." This is the power to keep people from ever thinking that something is a problem that should be dealt with. This is a power that is wielded more by society as a whole rather than by government. A good example of this would be a society in which women have a subordinated role but defend it because (for example) they believe that it is God's will that they play the role they do. Men, to Lukes, are exerting power here because they are creating a situation where the issue of making women equal never even comes up for consideration in the overt or covert faces of power.