It's denoted by a 1

• One easily bowled over?
• First thing usually hit by a bowling ball
• Lane's number one
• The front bowling pin in the triangular arrangement of ten pins
• #1 on a lane
• First of an alley's 10
• It's placed midway between the gutters

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
headpin \head"pin`\ (Bowling) n. The front pin in the triangular arrangement of ten pins. Syn: kingpin.

WordNet Word definitions in WordNet
n. the front bowling pin in the triangular arrangement of ten pins [syn: kingpin ]