- noun a colleague who holds the same position as you in another unit or sub-unit
- noun an opposite number, a partner. A military abbreviation used in police, underworld and more general contexts. Most users assume, understandably (and probably correctly), that this is based on ‘opposite’. A more fanciful suggestion is that it is in fact a corruption of ‘hop o’ my thumb’, rhyming slang for chum.
- noun an operation, particularly a military operation (a surgical operation is simply an ‘op’)