What is an adjective that describes the property of conferring fitness?
Terms

A trait variant that is associated to a higher fitness than another variant is said to be beneficial or sometimes adaptive. On the other hand, a trait variant that is associated to a lower fitness is said to be deleterious or sometimes maladaptive.

"beneficial / deleterious" vs "adaptive / maladaptive"

The terms are very similar but I would tend to think that there are tendencies of using either "beneficial / deleterious" or "adaptive / maladaptive" depending upon the specific context.

These terms can also be used when talking about genetic variants and not only phenotypic trait variants. In evolutionary genetics, the terms "beneficial / deleterious" are used more often than "adaptive / maladaptive".

Also, in general, the terms "adaptive / maladaptive" are often used in absence of variation for the trait in question in the population while the terms "beneficial / deleterious" are most often used when there is variation in the population.

Note

Note that trait X is a trait that confers fitness does not make much sense as is explained here.