A pretty straightforward one today but a matter worth resolving nonetheless.
In all English-speaking nations, 'astronaut' is the term used to describe anybody who visits space professionally. Cosmonaut, on the other hand, refers specifically to anyone who traveled outside Earth's atmosphere under the auspices of the Russian Federal Space Agency or its predecessor the Soviet Space Programme. By this definition, Helen Sharman, the first Briton in space, was a cosmonaut.
Another quick word about the remarkable Ms Sharman - before Tim Peake slipped Earth’s surly bonds, it wasn’t uncommon for Helen to be described as the only true Briton to have traveled in space.
To clarify this point, seven other people claiming British nationality have left our atmosphere. However, when Michael Foale, Piers Sellers, Nicholas Patrick, Gregory Johnson and Richard Garriott accomplished the feat, they did so as American citizens.
Meanwhile, space tourist Mark Shuttleworth holds dual South African-British, Semantics aside, we can at least confidentially confirm that Dame Helen (as she really ought to be by now) is Britain's only female astronaut. To date...