From The Jargon File (version 4.4.7, 29 Dec 2003):
tty
/T·T·Y/, /tit'ee/, n.
The latter pronunciation was primarily ITS, but some Unix people say it
this way as well; this pronunciation is not considered to have sexual
undertones.
1. A terminal of the teletype variety, characterized by a noisy mechanical
printer, a very limited character set, and poor print quality. Usage:
antiquated (like the TTYs themselves). See also bit-paired keyboard.
2. [especially Unix] Any terminal at all; sometimes used to refer to the
particular terminal controlling a given job.
3. [Unix] Any serial port, whether or not the device connected to it is a
terminal; so called because under Unix such devices have names of the form
tty*. Ambiguity between senses 2 and 3 is common but seldom bothersome.