I learned it in the context of signal processing, where it means the inverse of the Roman connotation (keep every tenth vs. destroy every tenth).

Unfortunately, the previously incorrect usage won and it doesn't mean that anymore, according to major dictionaries: "kill, destroy, or remove _a large proportion of_" is given as the first definition in the Oxford dictionary used by Google. ("Unfortunately" is, of course, for people who have the original meaning activated because of how the word looks and/or know the historical context and feel the tension with the contextual meaning.)

To add completion to this, decimation was a Roman Legion punishment that was really high up in terms of severity.

The Centuria was a roman military unit of 100 men (The centuria size/meaning actually varied over time) but when the decimation punishment was applied that would mean that (by draw) every 10th men in the centuria would be killed, here is the catch, the people of their own centuria had to kill their own mates in decimation.