The Free On-line Dictionary of Computing (30 December 2018):

hairy ball

    A result in topology stating that a continuous
   vector field on a sphere is always zero somewhere.  The name
   comes from the fact that you can't flatten all the hair on a
   hairy ball, like a tennis ball, there will always be a tuft
   somewhere (where the tangential projection of the hair is
   zero).  An immediate corollary to this theorem is that for any
   continuous map f of the sphere into itself there is a point
   x such that f(x)=x or f(x) is the antipode of x.  Another
   corollary is that at any moment somewhere on the Earth there
   is no wind.

   (2002-01-07)