What is the definition of “creature” and is it used consistently?

In the Starter set and Basic Rules, it mentions “character”, “non-player character”, “player character”, “monster”, “creature”, and maybe other terms I’ve missed. Which of these terms are interchangeable?

Some examples from the Basic Rules PDF:

  • Page 105: Conditions Appendix refers only to creatures.

  • Chapter 11: all spell descriptions refer to creatures, but occasionally other terms such as humanoids.