An aviary is a large cage, or a whole room of cages, built so that birds can live inside it. You meet the word at a zoo or a bird park: the birds can fly a little, hop, and hide, but they cannot leave. Some aviaries are big enough that people walk through them. A small cage on a balcony is not really an aviary — the word wants space. Though actually keepers also use it for a building that feels more like a house than a cage, as long as it was made for birds. It is a quiet, slightly bookish word; in everyday talk a child may just say bird cage or bird house.