How to Say "sameness" in Spanish
The Spanish word for “sameness” is “identidad” — C1 level.
English → SpanishC1
nounC1formal
the quality of being exactly alike

Examples
El principio de identidad es fundamental en la lógica.
The principle of identity (sameness) is fundamental in logic.
Se demostró la identidad entre las dos ecuaciones.
The identity (equality) between the two equations was demonstrated.
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