High school physics teachers were examined on their views regarding the importance of the knowledge of physical concept definitions. The views were contemplated against the status of definitions as it emerges from the Bloom’s taxonomy that placed definitions lowly in the hierarchy of cognitive activities. The teachers, when reasoning their views, considered the importance of concepts definitions within the subject matter knowledge, as well as for the classroom interaction. Despite of the high value attributed by the teachers themselves to the knowledge of definitions, the accumulated data demonstrate many difficulties and inaccuracies on the part of teachers in providing definitions. The recorded difficulties were classified and analyzed. It is argued that the deficiencies revealed in the teachers’ actual knowledge of this subject suggest a revision of the features of the currently employed physics instruction and especially with regard to the physics teachers’ training.