This paper describes the first steps towards the homogenization of the lambda Bootis group, leading to a concise definition of lambda Bootis stars: Population I, hydrogen burning, metal poor (except of C, N, O and S) A to F-type stars. The definition does not depend on phenomenological features, like flux depressions, colour excesses, vsini values, etc. Based on this new homogeneous catalogue with 45 lambda Bootis stars, we discuss classification criteria which can be used for a spectroscopic and photometric all-sky survey for lambda Bootis stars in the field and in clusters of different ages.