alpha wave

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A pattern of smooth, regular electrical oscillations in the human brain that occur when a person is awake and relaxed. As recorded by the electroencephalograph, alpha waves have a frequency of 8 to 13 hertz. Also called alpha rhythm.

Wiktionary

1. n. An electrical signal produced by the brain while the subject is relaxed.

WordNet 3.0

1. n. the normal brainwave in the electroencephalogram of a person who is awake but relaxed; occurs with a frequency of 8-12 hertz