What purpose does tag app serve?
It has almost 40 questions. Few of them I looked at made me feel like it's the same as applications tag. Does it make sense to make them synonyms?
An app is definitely an applications. It may present questions spanning many platforms like web-applications, desktop-applications, mobile-app, and so on. And on the grounds of my first sentence, yes: the app and the applications tags should be synonyms.
However, if one considers semantics, this may not be the right course of action. applications may refer to the the applying of different principles like things from design-patterns, architecture, or algorithms. In this case, making them synonyms may be counterproductive.
If Programmers.SE could survey the questions under applications and see what categories the majority fall into, a better informed decision can be taken