Same for the terms "selective advantage" and "selective disadvantage" which I intend to use synonymously. There are usages of each on Google Scholar, but do evolutionary biologists understand what is meant?
What is unclear to you?
If you could highlight what is unclear to you, or what exactly are the semantic issues in your opinion, that would help to really address your misunderstanding. In absence of that, I will just give you a general explanation of their meaning.
What does 'fitness advantage' mean?
If a group of individuals has a fitness advantage, it means that this group of individuals has a higher fitness over some other group of individuals. The other group, then has a fitness disadvantage.
If we grouped these individuals based on genetics, then the genotypes having a fitness advantage will likely raise in frequency in the population. If we used phenotypic trait to group individuals and if the trait is heritable, then the frequency of these phenotypic traits is also likely to raise in frequency in the population.