Software dev is full of obscure keywords that describe otherwise pretty simple or basic concepts you stumble upon in practice naturally and that you probably already understand.
singleton: a class/object that is designed to be single use, i.e. only ever instantiated with a single instance. Typically used when you use class/objects more for flow control or to represent the state of the program itself, rather than using it to represent data
immutable: read-only, i.e. unchangeable
dependency injection: basically when you pass a function or object into another function object, thereby extending their effective functionality, typically for modular code and to separate concerns.
Here’s one more of my favourite examples of such a keyword: memoization