Is It Preexisting or Pre Existing?

You can use pre- in ways that are redundant, but it's a valid prefix, and preexisting has its own meaning. For instance, if you want to describe dinosaurs in relation to humans, existing doesn't work, but preexisting does.

Is pre-existing one word or two?

Although the OED uses a hyphen in “preexisting,” as do all the examples cited for this sense of the term, we'll follow the hyphenless spelling in The American Heritage Dictionary of the English Language (5th ed.) ... In this sense, the OED defines “preexisting” simply as existing beforehand.

What does pre-existing mean?

: existing at an earlier time : existing before something else.