The word ‘aptitude’ is often misused to mean ability or achievement, and in the context of psychometric testing aptitude may be regarded as just another way of referring to specific ability. There is, however, a subtle technical difference between the three words ‘achievement’, ‘ability’ and ‘aptitude’, which can be distinguished as follows:
Achievement ^ what you have accomplished in the past.
Ability ^ what you are able to demonstrate in the present.
Aptitude ^ how quickly or easily you will be able to learn in the future.