European Patent Laws do not normally define an invention in positive terms; they rather exclude certain matters as not being an invention or as not being patentable.
Briefly though, it is considered that an invention consists in a solution to a technical problem.
In order to be patentable, the invention (in any field of technology, including chemistry, biotechnology, electronics, mechanics etc.) needs to be novel, to involve an inventive step and to be susceptible of industrial application.