In computer terminology, the name of a file. As far as your hard disk is concerned, everything you save is just a string of meaningless numbers; however, one luxury afforded you by your operating system of choice is the ability to give meaningful names to your data. Filenames (and the filesystem software which knows how to interpret them) are what allows you to refer to e2node.txt (or whatever) instead of having to remember that you saved your latest writeup starting at block number 142857 on your second hard drive.