(thing) by jimbeam Mon Sep 25 2000 at 11:48:03
1. Computing:
An abbreviation (or, if you will, an ETLA) of Man In The Middle.

Examples:

"{The Freenet Project will use} a Public Key Infrustructure to avoid MITM attacks."
"The encrypted link helps reduce MITM (Man-In-The-Middle) attacks such as Node Spoofing, Session Hijacking, etc."

2. Meteorological:
Magnetospheric, Ionospheric, Thermospheric, and Mesospheric
(source of (2): NASA)
I like it!