Welcome to MonkeyVision™.
For anyone wondering, a meme is an idea that, like a gene, can replicate and evolve.
Think of it as a unit of cultural information that represents a basic idea that can be transferred from one individual to another and is subject to mutation, crossover and adaptation.
Simply put: "memes are the cultural and idealogical counterpart to genes".
This is probably beyond a meme at this point since so many mashups exist out there, however the latest version of this meme refers to software APIs (Application Programming Interfaces) rather and music or video mashups. A good overview of the "new" mashup can be read HERE.
Two good sources to follow the evolution of this meme/tactic are Programmable Web and Google Maps Mania Blog. The former being a complete resource including a database of APIs available to "mash" and the latter focusing on what creative folks and marketeers can do with a single API.