a (geometry) Perpendicular to a tangent line or derivative of a surface in Euclidean space.
a (algebra) (Of a subgroup) whose cosets form a group
a (algebra) (Of a field extension of a field K) which is the splitting field of a family of polynomials in K
a (probability theory|statistics) (Of a distribution) which has a very specific bell curve shape
a (complex analysis) (Of a family of continuous functions) which is pre-compact
a (set theory) (Of a function from the ordinals to the ordinals) which is strictly monotonically increasing and continuous with respect to the order topology
a (linear algebra) (Of a matrix) which commutes with its conjugate transpose
a (functional analysis) (Of a Hilbert space operator) which commutes with its adjoint