egg timer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A small hourglass running three to five minutes, used for timing the boiling of eggs.

Wiktionary

1. n. A small version of an hourglass used to time the boiling of an egg.
2. n. by extension A timer that can be set to sound an alarm after a certain amount of time; a kitchen timer.

WordNet 3.0

1. n. a sandglass that runs for three minutes; used to time the boiling of eggs