The part of a snare drum that has several small strings or catgut threads strung tightly across it. The snare head is not struck but makes a distinctive resonant sound, the timbre of which can be changed by tightening or loosening the snares.
"Their drums...received several small shot in the batter heads, which they went through, but immediately struck out again by the Rims, and touch'd not the snare heads." (1704, E. SMITH, _Athenian Oracle_ III. 423, cited in OED)
"‘Snares’...consist of a number of gut or wire strings stretched across the lower skin or ‘snare head.’" (A. C. BAINES, _Musical Instruments_ xiv. 335, 1961, cited in OED)