Vector which is perpendicular to said surface or manifold. In a inner product space, the inner product of the normal vector with all vectors which comprise the surface is 0.
... Gould
1986 Gordon Lightfoot
1987 The Guess Who[?]
1989 The Band
1990 Maureen Forrester[?]
1991 Leonard Cohen
1992 Ian and Sylvia[?]
1993 Anne Murray
1994 ...