gemination (orthography)
Definition - The doubling of a written letter.

Example -

Etymology -
The term derives from the Latin geminare, to double.

Oxford English Dictionary -
Its first citation is from 1877:
"Gemination is the doubling of a consonant … A real gemination can not occur at the beginning or at the end of a word … nor is it easy after a long vowel."
(March Comp. Gram. Anglo-Saxon 16)

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