| A training bra is a brassiere designed to be worn by young girls whose breasts are just beginning to develop, but are not yet large enough to fit a "standard" bra. A solid definition of the species by size is difficult to make - I have seen 34A proposed as the cutoff point, or perhaps AA or lower cup size, but bras worn by mature women with breasts of this size are not considered "training bras", and these woman often avoid bras marketed as such, which tend to be frilly, flowery, and otherwise designed for a pre-adolescent demographic.
While bras of these sizes don't provide much support, breasts of these sizes don't particularly need it, and shaping effects would be of limited utility. Instead, the training bra serves to familiarize its wearer with the feel of wearing bras, the processes of securing and removing them, the way they affect the clothed form and appearance, and the like. Further, the bras also reaffirm their wearers' femininity, an effect whose importance should not be underestimated in young girls anxious about puberty and the beginning of "womanhood", or those who may feel unwomanly and insecure for having smaller breasts than their peers. | 
