[MaysonL]

That's omega, which is used as a symbol for ohms. It's also [and originally] the last letter in the Greek alphabet.

[lloeki]

Also, Ohms measure impedance, and an impedance value is noted Z.

Most of the alt/alt+shift combinations on the Mac keyboard layout make sense.

[nspragmatic]

I see, thanks. I originally wrote omega (I knew it was the last letter, alpha/omega and such), but changed it after checking the character viewer in OS X, which called it an ohm.

[duskwuff]

Capital letter omega is U+3A9: Ω

Ohm sign is U+2126: Ω

They probably look identical in most (if not all) fonts, but they're treated as two separate characters in Unicode.