I use MathWorld too but it's limited for my purposes, which includes conceptual understanding: Its articles tend toward strict definitions and not concepts, some terms I lookup are omitted, and unless you already are familiar with the term defined then it can be hard to comprehend.

Compare, for example:

* http://mathworld.wolfram.com/ModelTheory.html

* http://plato.stanford.edu/entries/model-theory/

EDIT: Replaced the examples with better ones

Scholarpedia is another good and authoritative source. The articles are typically written by the person who created the concept, or an active authority: http://www.scholarpedia.org

Indeed, one of the most authoritative sources. They should expand the fields treated, imo.

How does mathworld stack up against wikipedia for Math? I often wondered this.

Here's a review of Wikipedia's math articles by a mathematician:

http://wikipediocracy.com/2013/10/20/elementary-mathematics-...

If you don't already know what you are reading, you can't distinguish knowledge from nonsense.