Definition - Multiple letters that have been joined together to form a single letter.

Example -
œ and æ

Etymology -
The word entered English from Old French in about 1400. At that time it denoted "something used in tying or binding." It ultimately derives from the Latin ligatus, the past participle of ligare, to bind.

Oxford English Dictionary -
Its first citation in this sense is from 1693:
"These Ligatures have been a long time Thorns in the Eyes of all that first learn Greek."
(Phil. Trans. XVII. 887)

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