What is Brønsted acid and base, acid–base couple?

A Brønsted acid is a species able to give a hydrogen ion $\ce{H+}$; a Brønsted base is a species able to take one. An acid HA and the base $\ce{A-}$ it becomes by losing $\ce{H+}$ form an acid–base couple, written $\ce{HA}$/$\ce{A-}$: $\ce{HA} \rightleftharpoons \ce{A-} + \ce{H+}$.