When you see a work written by Mozart what does K stand for?

The letter K in the numbering of Mozart's works stands for "Köchelverzeichnis", which is the name of the catalogue of Mozart's works compiled by Ludwig von Köchel. Köchel's catalogue was first published in 1862 and has been revised and expanded several times since then. It is the standard reference for identifying and cataloguing Mozart's works, and the K number is used to refer to each work in the catalogue. For example, Mozart's Symphony No. 40 is known as Symphony No. 40 in G minor, K. 550.