An ELT Glossary : Backwash / Washback
Definition : Backwash (also called washback) is the effect that knowledge of the contents of a test may have on the course which precedes it. It may be positive or negative.
Example : If students are working towards an exam where all of the test items focus on grammatical accuracy, the teacher (possibly at the students instigation) may spend much of the preceding course focusing on this area - and possibly neglecting other areas that they will need outside the course such as spoken fluency or listening. Thus the test would have negative backwash - it pushed the teacher into "teaching for the test" rather than providing a balanced course which dealt with all the students needs and developed other areas of competence than just grammatical knowledge.