- A sentry or guard.
- (computer science) a unique phrase of characters (a string), used to mark a document for processing by a computer program, also called a keyword. For example, an HTML tag such as '<table>' is a form of sentinel as its a unique phrase that is non-reaccuring in the english language. However the use of '<' and 'table' do occur in english but not in this form, so '<table>' is a relatively good sentinel phrase.
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