Theme entry
A theme entry is one of the small number of long answers in a crossword that share a hidden idea, and around which the rest of the grid is constructed.
Theme entries are normally the longest answers in the puzzle and are placed symmetrically, so a solver who spots one can often guess where the others sit. The shared idea might be a pun pattern, a category, a phrase with a word hidden inside it, or a transformation applied to a familiar expression. Some puzzles include a revealer, a final theme entry that states the trick outright.
Construction runs in the opposite order to solving. A constructor chooses the theme entries first, places them in a symmetric arrangement, and only then fills the remaining squares with ordinary words that have to interlock around fixed letters. That is why the theme constrains everything — a bad theme set makes the rest of the grid impossible to fill cleanly.
A puzzle with no theme is called a themeless, or a freestyle in America, and it trades the shared idea for long interlocking answers and a harder clue set. In American daily papers, the early-week puzzles are usually themed and the end-of-week ones usually are not.
Daily Mini Crossword — a five-by-five grid — too small for theme entries, which is part of why it is quick.