Definition of "sentinel" [sen•ti•nel]

• One that keeps guard; a sentry. (noun)
• To watch over as a guard. (verb-transitive)
• To provide with a guard. (verb-transitive)
• To post as a guard. (verb-transitive)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "I do have a veterinarian friend who has what he calls a sentinel cat."
• "Lieutenant Hayward treated him with a sort of contemptuous look, and began to enter into conversation with him respecting the _Bounty_, but I called the sentinel in to take them into custody, and ordered Lieutenant"
• ""Oh, this isn't a co-op exactly, and it's not like, uh, the Guardian Angels, except for what they call sentinel stance, and - It's sort of like a church, except not that either."