What does the prefix or mean?

The prefix "or" means "one or the other". It is used to indicate a choice between two or more options.

Here are some examples:

* Orchard: A place where fruit trees are grown (or a place where trees are grown).

* Ornithology: The study of birds (or the branch of zoology that deals with birds).

* Orthogonal: Perpendicular (or at right angles).

* Otherwise: In another way (or under different circumstances).

Note: The prefix "or" is often used in conjunction with the word "either" to emphasize the choice between two options: "either or".