1.nounA leader or head of a group of people, organisation, etc.
"All firefighters report to the fire chief."
2.nounThe top part of a shield or escutcheon; more specifically, an ordinary consisting of the upper part of the field cut off by a horizontal line, generally occupying the top third.
3.nounThe principal part or top of anything.
4.nounAn informal term of address, sometimes ironic.