The front bowling pin in the triangular arrangement of ten pins

• One easily bowled over?
• It's denoted by a 1
• First of an alley's 10
• It's placed midway between the gutters
• #1 on a lane
• First thing usually hit by a bowling ball
• Lane's number one

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.

Wiktionary Word definitions in Wiktionary
n. 1 The kingpin (in ten-pin bowling). 2 A straight metal wire with a flat end used for beads in jewelry making.