Weber's concept of value-agnostic rational action motivated by pure efficiency. Contrast with value rationality, which is concerned with deliberate formulation of the values governing action.
Weber believed that instrumental rationality was inevitably more efficient and powerful, being not weighed down by anything as unwieldy as values, and therefore destined (in the absence of countermeasures) to dominate other kinds of rationality. Rationalization is the name for this process of instrumental rationality crowding out every other kind of rationality.