Okay, it took me forever to find the OMG's formal UML Specification v1.5, so here's a direct link to the page it is on. I'm not sure why, but I started at www.omg.org (don't go to omg.org without the "www" prefix because that doesn't work), and they bounced me to www.uml.org, and they bounced me back to www.omg.org for the actual doc. Argh.
By Alvin Alexander. Last updated: June 3 2016