Like one that has been chosen as the official definition by the scientific authorities or something like that?
I don't think there is any, this is ridiculous. I needed one for something I'm working on and this is the closest one to a scientific definition (i.e. it can be consistently used) I know of:
A collection of recorded theories that follow the scientific method, some of which may contain some truth about a particular aspect of nature.
But I can't use where I got this from as a reference because there is no scientific consensus on it. Sillyness!