Abstract: A luminaire (1) comprises lamp holder (3) and a lamp (2) mountable to the lamp holder (3). The lamp holder (3) has a body (6) with a cylindrical inner surface (8) and protuberances (1) located around the cylindrical inner surface (8). The lamp has a housing (15) with faces (21) and groups of grooves (22a, 22b, 22c) around an outer surface. Each group of grooves (22a, 22b, 22c) is spaced away from a first end of the housing (15) by a different distance. The faces (21) facilitate insertion of the housing (15) into the body (6) at a first orientation by providing space for the protuberances (10) between the housing (15) and the cylindrical inner surface (8). The lamp (2) is rotatable in the lamp holder (3) from a first orientation to a second orientation in which the grooves of one of the groups of grooves (22a, 22b, 22c) accommodate the protuberances (10) to secure the lamp (2) to the lamp holder (3).