A tombolo, from the Italian tombolo, derived from the Latin tumulus, meaning 'mound,' and sometimes translated as ayre (Old Norse eyrr, meaning 'gravel beach'), 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. Several islands tied together by bars which rise above the water level are called a tombolo cluster. Two tombolos can form an enclosure (called a lagoon) that can eventually fill with sediment.

"True" tombolos are formed by wave refraction. As waves near an island, they are slowed by the shallow water surrounding it. These waves then refract or “bend” around the island to the opposite side as they approach. The wave pattern created by this water movement causes a convergence of longshore drifting on the opposite side of the island. The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there, conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up, the beach shoreline, known as a spit, will connect with an island and form a tombolo.