Tangent (noun)

1. A straight line or plane that touches a curve or curved surface at a single point, but does not intersect it at that point.
2. A deviation or departure from a course or subject, especially in a conversation or discussion.

Origin:

Early 17th century: from latin tangent- "touching", from the verb tangere.

Examples:

1. The tangent to the circle at the point was parallel to the x-axis.
2. The lecture quickly took a tangent and became irrelevant.
3. The mathematician studied the tangents of various curves.
4. The conversation took a tangent and started discussing the weather.
5. The tangent line provided important information about the curve at that point.