Semantic definitions of full-scale programming languages are rarely given, despite the many potential benefits. Partly this is because the available metalanguages for expressing semantics usually either L(A)T(E)X for informal mathematics or the formal mathematics of a proof assistant make it Much harder than necessary to work with large definitions. We present a metalanguage specifically designed for this problem, and a tool, Ott, that sanity-checks Such definitions and compiles them into proof assistant code for Coq, HOL, and Isabelle/HOL, together with L(A)T(E)X code for production-quality typesetting, and OCaml boilerplate. The main innovations are (1) metalanguage design to make definitions concise, and easy to read and edit: (2) an expressive but intuitive metalanguage for specifying binding structures; and (3) compilation to proof assistant code. This has been tested ill Substantial ease Studies, including modular Specifications of calculi from the TAPL text, a Lightweight Java with Java JSR 277/294 module system proposals, and a large fragment of OCaml (OCaml(light), 310 rules). with mechanised proofs of various soundness results. Our aim with this work is to enable a phase change: making it feasible to work routinely, without heroic effort, With rigorous semantic definitions of realistic languages.