Acronym for Unified Modeling Language. You might also be looking for UserModeLinux.
A standardisation of a super-set of all diagramming conventions for all mainstream software development methodologies along with a standardisation of file formats for exchanging diagrams. In a nutshell, pretty pictures for computer scientists.
Has lead to a dramatic improvement in the sales and InterOperability of SoftwareDevelopmentTools. Has not lead to an obvious improvement in the quality of software.