Single trip insurance covers you for a single return journey to a specified location. It is usually purchased with defined dates for start and end of a single trip and the policy terminates at the end date so specified unless extended. On the other hand, a multi-trip insurance is an annual policy that covers you for multiple trips abroad during a given year.
If you are a frequent traveller, you may want to compare prices and features for a multi-trip product that provides you ongoing cover for each of your overseas trips during a one year period. These products, however, usually restrict coverage for the duration of each trip during the year to a specific number of days.