What is Brønsted acid and base, couple?

A Brønsted acid is a species able to give a proton $\ce{H+}$; a Brønsted base is a species able to accept one. An acid HA and the base A that it becomes by losing a proton form an acid–base couple HA/A, related by the formal half-equation $\mathrm{HA} \rightleftharpoons \mathrm{A} + \ce{H+}$. In water the proton is never free: it is carried by a water molecule as $\ce{H3O+}$.