This is a 3-terminal microswitch, with a small push-button.
This switch has a great 'clicky' sound to it, and a nice tactile feel.

 

Features:

• Max 4A 125/240VAC
• 2 mounting holes
• Long mechanical operating life
• Contact resistance: less than 0.2Ω
• Insulation resistance: 500Ω minimum @500VDC
• Rated to 10000 clicks

Dimensions:

• 0.63x1.1x0.42"
• Weight: 5g