Reliable mathematical term (8)

I believe the answer is:

constant

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'reliable mathematical term' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe there's an association between them I don't understand?

This is all the clue.

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(Other definitions for constant that I've seen before include "Remaining the same" , "Faithful - unchanging" , "Steady and unceasing" , "Endless" , "sustained" .)

I've seen this clue in The Globe and Mail.

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