You could find one answer here: http://en.wikipedia.org/wiki/Stack_(data_structure)

In short: a data structure where the last thing you put in is the first you get out.

To others it means everything you can pile up in one or the other way. Like MySql on Linux with Apache and PHP which gives us the LAMP stack ...

To some weird people it might mean this: http://www.rocktownhall.com/blogs/media/blogs/rth/MarshallSt...