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training bra

n. A type of bra used by girls, usually teenage girls when their busts are still small.

Wikipedia
Training bra

A training bra (also trainer bra or bralette) is a lightweight brassiere designed for girls who have begun to develop breasts during puberty when the girl's breasts are not yet large enough to fit a standard-sized bra. Breast growth is usually defined by the Tanner scale. Girls who are developing breasts are at Tanner stage I and II. Training bras are usually a lightweight, unlined, pullover style bra and feature a soft, elastic bra band and soft bra cups. Prior to the marketing of training bras, a pre-teen or young teen girl in Western countries usually wore a one-piece "waist" or camisole without cups or darts.

When a girl receives her first bra, it may be seen as a long-awaited rite of passage in her life signifying her coming of age. Bras for pre-teens and girls entering puberty were first marketed during the 1950s. Some companies have been criticized for marketing bras to pre-teens and of sexualizing girls at an early age.