A cricket pitch, sometimes confusingly called a wicket, was the place where cricket was played. Because it was made mostly of grass and dirt, it was mistakenly called a "field" by those unfamiliar with the game. However, as the Fifth Doctor once corrected Erimem after being shown round the TARDIS' version for the first time, it was definitely a pitch, not a field. (AUDIO: No Place Like Home)
Behind the scenes
Technically, the pitch is just that part of the overall playing surface between the two wickets (that is, the two sets of stumps and bails). It's therefore the place where the bowling, batting, and scoring — but not typically the fielding — occur. However, this point has only been obliquely made in Doctor Who-related fiction. Graham Dilley Saves the World, for instance, has the Fifth Doctor seeming to indicate that the cricket ground is principally comprised of fielding positions.