Col. is how you write colonel when you do not want the whole word — usually before a family name, as in Col. Chen. A colonel is a senior army officer, high up but not at the very top. You use Col. on envelopes, nameplates, captions, and signatures. You do not say “Col.” out loud as three letters; you still say the full rank. Though actually, that full word is a famous spelling trap: colonel sounds like kernel, not like colonial. English kept one spelling and another sound, and nobody cleaned it up. In writing, Col. can also stand for other things (a newspaper column, the US state Colorado, sometimes college), but this entry is only the rank.