1. A guard, especially a soldier posted at a given spot to prevent the passage of unauthorized persons.
2. The duty of a sentry; watch.

[Perhaps alteration of obsolete sentrinel, variant of SENTINEL.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.