Share-alike is a term invented by Creative Commons. They describe it, in plain English, as "If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one." 
The context of "alter, transform, or build upon" is that which creates a derivative work, as opposed to that which creates a collective work. Creative commons has created its own, international, definitions, using the terms "adaptation" and "collection", but the distinction roughly mirrors the US concept of derivative work vs. collective work (see ).
The concept of share-alike embraces and extends the earlier concept of "copyleft". The term "copyleft" is defined by the FSF as "a general method for making a program (or other work) free, and requiring all modified and extended versions of the program to be free as well."