Crossword Genius

A term for mother (5)

Ross

I believe the answer is:

mater

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'a term' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'mother' is the second definition.
(old-fashioned term for mother)

'for' is the link.

Can you help me to learn more?

(Other definitions for mater that I've seen before include "An old Roman mother" , "Mum" , "See 12" , "Mother (Latin or old slang)" , "One marrying" .)

I've seen this clue in the Evening Standard.
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