(1) That which has been learnt through study or practice.
[Van Dale Hedendaags Nederlands]
(2) Knowledge is the the ability to perform a particular task. This ability is a metaphorical function of the Information, Experience, Skills and Attitude a particular person possesses at a particular moment in time: K=Fn(I.EVA).
[Weggeman, 2000]
(3) The terms data and knowledge are often used interchangeably without a a clear definition, resulting in the lack of a critical distinction.
An informal and highly practical way of describing and distinguishing between both concepts is to present them as two extremes on a scale of complexity . Data is found on the low end of the complexity scale, and knowledge on the high end. This concerns the complexity of symbolic models (representation) that are needed to express something: it is easy to express the length of an object (data), the same cannot be said of an eco-system (knowledge).
You could also say: knowledge is data plus interpretation.
[Obrst, Liu, 2003]