R, or greek OMEGA
AnswerAre you referring to the circuit symbol for resistance, or the quantity symbol?
The circuit symbol depends on in which country you live. In the United States, the symbol is a zig-zag. In Europe, the symbol is a rectangle.
The quantity symbol for resistance is the italicised capital letter, R.
On the other hand, the SI symbol for the unit of measurement of resistance, the ohm, is the capital Greek letter, omega.