1.
paper (in rock-paper-scissors); open hand
The open-hand shape in the game janken, which beats rock (グー) and loses to scissors (チョキ). Also used for an open, spread hand generally.
One of the three hand shapes in じゃんけん (rock-paper-scissors), together with グー (rock) and チョキ (scissors). パー is a flat open hand; it beats グー and loses to チョキ.
Common collocations
Usage
In casual speech パーになる separately means "to come to nothing" (e.g. plans falling through), a different, idiomatic sense.