[_delirium]

It's from classical Greek/Latin rhetoric. In at least some grammarians' writings, there are grammatical units called the 'period', 'colon', and 'comma' (in decreasing order of granularity). The names became applied to punctuation separating those units as well.

Although, confusingly, I believe the English semicolon functions more similarly to the punctuation associated with a Greek/Latin colon.