Tangent line?

ALSO

Think of a word that means "in addition" and sounds like a math term for a line touching a curve.

This clue plays on the double meaning of "tangent." In geometry, a tangent is a line that touches a curve at a single point, but in conversation, going off on a "tangent" means veering into an unrelated topic—often introduced with the word "also." The answer "ALSO" cleverly hints at that shift in discussion, turning a math term into a playful nod to how conversations drift.