What is the difference between 'single-entry' and multiple-entry' visas?

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Depending on the reciprocal treaties governing visa issuance for each nationality, a visa may allow one entry before its expiration, or it may permit the traveler to enter at various times. A single-entry visa may only be used for one entry for the specific purpose for which it was granted. In general, the number of times that a multiple-entry visa may be used depends upon restrictions of time (expiration date of the visa), the nationality of the traveler, or restrictions placed on the total number of days permitted in country during any year.