Cocoa butter, the fatty part of chocolate, can solidify into different crystalline forms at different temperatures. This means that untempered chocolate is vulnerable to changes in temperature. Chocolate must be tempered to stabilize it before use, or crystals may form, causing what is called "bloom". This can appear as a white film on the surface of chocolate, and in some cases the chocolate may become soft or crumbly. While bloom is unwanted, it is not harmful and chocolate with bloom is still safe to eat.

This definition is in context to Chocolate. See more contextual defintions for Bloom.