It is a complex and fraught subject, SLF3. Do look at other discussion threads here - there are many. They will help you find the legal definition but, crucially, many of the get outs that airlines try to use. Firstly, they ALWAYS rebuff a claim. This is because many then give up. The key definition is the delay of arrival after the scheduled time.
As a point to the future, if delayed, it can be very useful to take a photograph of the arrival board, or to do so online as soon as you get home.