The Whiskered Tern show similarities in appearance to both the Common Tern and and to the Black Tern.
They are distinctive in breeding plumage, with a black cap, extensive white cheeks, a grey breast and sides with a darker grey patch on the belly, and with white under the tail up to the legs. 
In winter, the forehead becomes white, the bill black with a reddish tinge at the base and the body plumage a much paler grey.
The crown is flecked with white in the juvenile and in the wintering adult (3rd photo).

Mila 23 area, Danube Delta, Romania, 23 & 24 May 2018