tan·gent 
/ˈtænʤənt/ noun
plural tan·gents /ˈtænʤənt/ noun [count] geometry : a line that touches a sphere or circle at only one point
go off on a tangent (US) or Brit go off at a tangent
: to start talking about something that is only slightly or indirectly related to the original subject
▪ She went off on a tangent about what happened to her last summer.
▪ She went off on a tangent about what happened to her last summer.
