Catchword is also a name for a headword

Headword

A headword, head word, lemma, or sometimes catchword is the word under which a set of related dictionary or encyclopaedia entries appear. The headword is used to locate the entry, and dictates its alphabetical position...

 in a dictionary.

For the game show, see Catchword (game show)

Catchword (game show)

Catchword was a daytime word game show first shown on BBC1 Scotland from 17 April 1985 until 2 April 1986, hosted by Gyles Brandreth, and then network on its sister channel BBC2 from 5 January 1988 until 23 May 1995, hosted by Paul Coia-Format:...

.

A catchword is a word

Word

In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

 placed at the foot of a handwritten or printed

Printing

Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....

 page that is meant to be bound along with other pages in a book. The word anticipates the first word of the following page. It was meant to help the bookbinder or printer make sure that the leaves were bound in the right order or that the pages were set up in the press in the right order. Catchwords appear in some medieval manuscripts, and appear again in printed books late in the fifteenth century. The practice became widespread in the mid sixteenth century, and prevailed until the arrival of industrial printing techniques late in the eighteenth century.

Theodore Low Devinne's 1901 guide on Correct Composition had this to say:

For more than three centuries printers of books appended at the foot of every page the first word or syllable of the next page. This catchword was supposed to be needed by the reader to make clear the connection between the two pages ;  but the catchword is now out of use, and it is not missed.