A cage was a type of prison or secured area where animals were held.
Doctor Phlox held several of his animals in sickbay aboard Enterprise NX-01 in cages, including his Pyrithian bat. (ENT: "The Catwalk", "Regeneration")
A white cage was one of the items Garos kept in his curio shop on the Akaali homeworld in 2151. (ENT: "Civilization")
In 2266, in response to James T. Kirk's admiration for Doctor Tristan Adams's theories about treatment of penal colony inmates, Leonard McCoy affirmed that "a cage is a cage, Jim." (TOS: "Dagger of the Mind")
In 2269, Christine Chapel reported that a group of gossamer mice in the USS Enterprise's medical lab were becoming too small for the cage mesh to prevent their escape. (TAS: "The Terratin Incident")
On Betazed, women used to wear huge wigs that incorporated cages for tiny animals. These cages were kept in large holes in the middle of the wigs. This fashion was very cruel for the animals and was abolished after many years as a result of one woman's defiance. (TNG: "Half a Life")
In 2366, a cockatoo in a cage was part of Reginald Barclay's holographic program. (TNG: "Hollow Pursuits")
In 2383, Nandi kept an animal in a cage on the Damsel. (PRO: "First Con-tact")
A symbolic cage was referenced in the final draft script of TOS: "Miri", as a classroom full of Onlies surrounding Kirk was likened to a cage containing "too many" lions and a lion tamer.