Of Latin etymology : cum- , together with , in company of , with and the participle present efficiens efficientis de efficicio , carry out, execute, cause , produce : What together with another generates something. Mathematical expression understood as numeric value, multiplicative factor or relationship between two magnitudes. So for example, in Psychology we talk about IQ, although it would be better to say IQ, which would be determined by the mental age (expressed according to the correct answers to the well-made intelligence tests) divided by age chronological of the child. The added -a- I do not know what it might mean.