breast - bust - bosom
A woman's breasts are the two soft, round pieces of flesh on her chest that can produce milk to feed a baby.
...a beggar girl with a baby at her breast.
...women with small breasts.
A woman's breasts can be referred to as her bust, especially when you are talking about their size. Note that bust refers to both breasts together. You do not talk about a woman's ‘busts’.
She has a very large bust.
Bust is also used to talk about the measurement around the top part of a woman's body at the level of her breasts.
‘Bust 34’ means that the garment is a size 12.
A woman's breasts can also be referred to as her bosom /^bʊzəm/. This is an old-fashioned or literary word.
...hugging the cat to her bosom.