The second edition of the widely acclaimed Encyclopedic Dictionary of Mathematics was published in 1987 and is now available in paperback. It includes 70 new articles, particularly in applied mathematics, expanded explanations and appendices, coverage of recent work, and reorganization of older topics.
I fished my two-volume hard-back set out of the garbage when I worked at the Half-Price books warehouse in my mid-twenties. It had one little ding, or I guess one big ding on one volume, I have that volume in paperback as well with an even smaller ding. For advanced mathematical concepts and just to be able to look at the fundamental equations I have never needed much more than this, though I'm no mathematician. This has been a good set to have as an artist type person. It doesn't have a bunch of applied science mathematics, like say optics math or electronics math, but all of that math comes out of pure math principles which this set archives with historical contextualisation. I'm just happy I saved their lives. Two big beautiful MIT editions.