I'm quite positive that I saw before an encyclopedia (more accurately, dictionary/glossary) on Elsevier website. I remember I looked up the definition of a hyperplane there, few months ago. But I can't find it through Google anymore.
Does anyone know the link?
On
What you are looking for doesn't seem to exist (if you go to Elsevier site and type any combination of what you are looking for, like math glossary, you don't get any relevant answers).
You might want to check Wolfram Mathworld: http://mathworld.wolfram.com/
You may be confusing Elsevier with Springer. The Springer Online Encyclopaedia of Mathematics is quite an impressive reference.