A metabolic alkalosis is a primary acid-base disorder which causes the plasma bicarbonate to rise to a level higher than expected. The severity of a metabolic alkalosis is determined by the difference between the actual [HCO3] and the expected [HCO3].
Secondary or compensatory processes which cause an elevation in plasma bicarbonate should not be confused with the primary processes. An elevation in bicarbonate occurring in response to a chronic respiratory acidosis should be referred to as a 'compensatory response' and never as a ‘secondary metabolic alkalosis’.
You should be aware that many articles (esp in the surgical literature) will refer to a 'compensated metabolic alkalosis' as a 'metabolic alkalosis with a (secondary) respiratory acidosis'. This is wrong as the hypoventilation is a compensatory process and does not indicate any primary respiratory problem. Another implication of the incorrect terminology is that acid-base disorders always occur in pairs and this is ridiculous and of no help in patient management.
The terminology of acid-base disorders is covered in Section 3.1.