A sheaf is a bundle or cluster of objects. The unmodified term is normally considered equivalent to a garb, and this is sometimes made explicit (e.g., “a sheaf of wheat”). But in blazonry, the term also refers to a group of three charges, two in saltire surmounted by another palewise. The term is most often used with arrows: a “sheaf of (three) arrows” is a common motif, found in the arms of Joskyn, c.1435 [DBA1 11]. But the term has also been used in mundane blazons for charges other than arrows [Franklyn 302], and it has been adopted for use in Society blazons as well. (It thus replaces older, more awkward neologisms, such as “in estoile” or “in gyronny”).
The illustration shows a sheaf of three spears. It is also possible to have more than three charges in a sheaf, but such cases must be explicitly blazoned.
Aidan of Kilkenny bears: Argent, three sheaves of arrows sable flighted vert.
Conaire Anluan MacMurchadha bears: Vert, a sheaf of three spears argent within a bordure checky vert and Or.
Kilic ibn Sungur ibn al-Kazganci al-Turhani bears: Sable, a sheaf of five swords argent within an orle Or.