Competition
Rivalry among sellers (and sometimes buyers) that limits prices and encourages quality and innovation.
Competitive markets are a normative ideal in much economics, though perfect competition is a model, not a photograph of reality. Policy aims to protect the competitive process even when outcomes are unequal.
Links to deregulation
Some deregulation aimed to increase competition by removing entry controls; other reforms consolidated industries. Evaluating which happened requires data, not slogans.