The Q wave is the first downward deflection after the P wave and the first element in the QRS complex.

When the first deflection of the QRS complex is upright, then no Q wave is present. The normal individual will have a small Q wave in many, but not all, ECG leads. 

Abnormalities of the Q waves are mostly indicative of myocardial infarction and discussed further inside the relevant sections of ECG Reviews and Criteria. The terms “Q wave myocardial infarction” and “non-Q wave myocardial infarction” are earlier designations of different types of MIs ultimately resulting in, respectively, Q wave development or the absence of Q wave development.