I had to look it up but I'm still not sure I know..
Is it the items you use in the mosaic?
Tesserae-- anything you glue to a substrate to make a mosaic... I'm sure the dictionary probably says tiles, but I think the word has expanded to mean just about anything, wood, pebbles, beads, glass, stones, etc.
Here's a good description of tess . . .
Here is a link that might be useful: Wikipedia definition
Thanks that helps a lot, now I can talk like I know what I'm talking about.. :)
If you need to find anything about mosaic terms the following link is very useful.
Here is a link that might be useful: The mosaic dictionary