A tombolo, from the Italian tombolo, derived from the Latin tumulus, meaning 'mound', and sometimes translated as ayre, is a deposition landform in which an island is attached to the mainland by a narrow piece of land such as a spit or bar. Once attached, the island is then known as a tied island. A tombolo is a sandy isthmus.

Several islands tied together by bars which rise above the water level are called a tombolo cluster. Two or more tombolos may form an enclosure (called a lagoon) that can eventually fill with sediment.

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