The lozenge in heraldry is a diamond-shaped charge (an object that can be placed on the field of the shield), usually somewhat narrower than it is tall. It is to be distinguished in modern heraldry from the fusil, which is like the lozenge but narrower, though the distinction has not always been as fine and is not always observed even today. A mascle is a voided lozenge-- that is, a lozenge with a lozenge-shaped hole in the middle-- and the rarer rustre is a lozenge containing a circular hole. A field covered in a pattern of lozenges is described as lozengy; similar fields of mascles are masculy, and fusils, fusily.

The lozenge has for many centuries been particularly associated with women as a vehicle for the display of their coats of arms (instead of the escutcheon or shield). In modern English and Scottish, but not Canadian, heraldry, the arms of an unmarried woman and of widows are usually shown on a lozenge rather than an escutcheon, without crest or helm. An oval or cartouche is occasionally also used instead of the lozenge for such women.

Married women, however, always display their arms on a shield (except peeresses in their own right, who use the lozenge for their peerage arms even during marriage).

The shield of a married woman (and the lozenge of a widow) may combine her own arms with the arms of her husband, either by impalement side by side or (in the case of a heraldic heiress in English heraldry, but not Scots) in the form of a small "escutcheon of pretence" displaying the wife's arms over a larger shield (or, in the case of a widow, lozenge) of her husband's arms.

As a result of rulings of the English Kings of Arms dated 7 April 1995 and 6 November 1997, married women in England, Northern Ireland and Wales and in other countries recognising the jurisdiction of the College of Arms in London (such as New Zealand) also have the option of using their husband's arms alone, marked with a small lozenge as a brisure to show that the arms are displayed for the wife and not the husband, or of using their own personal arms alone, marked with a small shield as a brisure for the same reason.

Divorced women may theoretically until remarriage use their ex-husband's arms differenced with a mascle.

The lozenge shape is also used for funereal hatchments for both men and women.