TANGENT

(CIRCLE) [noun] [C] - a straight line which touches but does not cut into a curveIf someone (esp. UK) goes off at a tangent/(US and ANZ) goes off on a tangent/(UK and ANZ) flies off at a tangent they suddenly change what they were talking or thinking about.It's hard to get a firm decision out of him - he's always going off at a tangent.