Euphemism

A euphemism is a word or phrase used to avoid referring to something considered unpleasant or embarrassing.

A euphemism for "euphemism" is "parliamentary language".

Common euphemisms
Commonly, sexual and excretory activities and related body parts are especially referred to using euphemisms:

Euphemisms and politically correct language
The distinction between euphemisms and politically correct language may not always be clear -  for example referring to people with disabilities as "differently abled".

Register
Looked at from another angle all the above could simply be regarded as examples of different registers. To take one of the examples above: defecate - go to the toilet. One could also add the colloquial word "shit" to the group.

One could well imagine circumstances where the use of "defecate" would result in howls of laughter or at least raised eyebrows. In truth, all of these these words might be appropriate depending on the register being used by the speaker or writer.