Small, compact swallow with dark (glossy dark blue) upperparts, smudgy gray cast on whitish underparts, and bright white rump. Very similar to Northern and Nepal House-Martins, with gray-smudged underparts intermediate between the clean white belly of the former and black sides of the latter. Sometimes mistaken for House Swift when seen from above, but the chunkier shape and broader wings of the swallow are distinctive. Often nests in bustling colonies on cliffs and under the eaves of human-made structures.