The tubular embedding of a Submanifold of another Manifold is an Embedding such that whenever , where is the unit Ball in centered at 0. The tubular neighborhood is also called the Product Neighborhood.
See also Ball, Embedding, Product Neighborhood
References
Rolfsen, D. Knots and Links. Wilmington, DE: Publish or Perish Press, pp. 34-35, 1976.