A mushroom is said to be cespitose (or caestipose) when it grows in dense clusters, with the stems fused together or packed right up against one another at the base. Clustered (or "growing in clusters" without a mention of cestipose) seems to mean the same thing in some works, but in this website it will mean growing very closely together, but not with all the stems arising from a common point of origin.
Imbricate means "tiled", and actually has two different meanings. In speaking of growth habits, it refers to shelf-like mushrooms that grow from the side of the substrate and come out right over one another. See the entry on wrinkles for the other meaning.
A fruiting body (or other fungal part) with determinate growth has a definite shape that it "wants" to end up in. If something gets in its way, it may end up distorted, but we can usually recognize it as "trying" to assume a certain form. Most of the mushrooms you know have determinate growth.
A fruiting body with indeterminate growth simply expands in all directions for as long as its growth spurt lasts. While it will have some regular features that make it recognizable, its shape will be determined by what it runs into and has to go around. These are the fungi that always end up engulfing and picking up small local objects, like leaves, small sticks, etc. (some determinate growth fungi do this, too).
