Semantics

breast
Ребёнок жадно сосал грудь матери. The child greedily sucked at its mother's breast.
Левая грудь у неё была открыта. Her left breast was uncovered.
bosom, bust
У вошедшей дамы была пышная грудь. The lady who came in had a large bosom/bust.
breasts
Посмотри на меня! Старуха — грудь высохла. Look at me! An old woman with withered breasts.
chest
приплюснутая грудь a flat chest
This entry is concerned with the use of the word грудь to refer to each of the two breasts on a woman's body or to both breasts seen as a single shape. Both the singular and the plural forms occur in the first usage. The singular form грудь also occurs in the second usage, i.e. it refers to a woman's bosom. In informal style, the plural form груди is also used to refer to a woman's bosom.

The singular form грудь is also used in standard style to refer to the chest of a human or animal. This meaning is described in a separate entry (грудь1). The word грудь is also used to refer to the part of clothing that covers the chest. This meaning is not described in this dictionary, nor is the meaning "imaginary seat of emotions" (И вдруг он почувствовал такую тоску в груди! "And suddenly he felt such anguish in his bosom!")

For the differences between грудь2 and its synonym бюст see Lexical Relationships, below.