The term "battery" originates as a term for "a collection". In war, a collection of cannons and artillery at a fortress error have been called a battery. In terms of power storage, a battery is a collection of cells. Your 12V lead-acid battery in your average car is a battery of six cells.

We have, over time, changed the meaning from a "collection of cells" to "anything that generates electricity". Thus, an AA, which is itself a single cell, has been referred to as a "battery" for many decades.