A lake was a body of water.
The Doctor and his companions visited many lakes in their travels. On Earth, they visited Lake Garda, (PROSE: Loop the Loup) Lake Geneva, (AUDIO: Mary's Story) Dunstanton Lake, (AUDIO: Prisoners of the Lake) Lake Silencio, (TV: The Impossible Astronaut) Lake Vortigern, (TV: Battlefield) Loch Ness (TV: Terror of the Zygons) Nor' Loch, (PROSE: The Many Hands) and Silent Pool (TV: The Unicorn and the Wasp)
On other planets, they visited Lake Emerson, (PROSE: The Year of Intelligent Tigers) Lake Mono (PROSE: Night of the Humans) and the Lake of Mutations. (TV: The Daleks) One planet had a lake infested with crocodiles. (COMIC: The Tests of Trefus) Romana used to visit Lake Abydos in her youth. (AUDIO: Neverland) An artificial lake was one of the attractions on Hedgewick's World of Wonders. (TV: Nightmare in Silver)
Not all lakes were what they seemed. One lake on an unnamed planet was actually Zero's TARDIS. (AUDIO: Spring) Another lake, located on Erratoon, was actually a segment of the Key to Time. (AUDIO: The Prisoner's Dilemma)