Words interchanged

What is the fallacy called when two different words are used as if they are synonymous?

Example:

P1: Everything that exists must have a cause.
P2: The universe exists.

C: The universe was created.

1& 2 are fine. But, then, in the conclusion "caused" was replaced by created, as if they are synonymous, but they're not. Is this a logical fallacy, or just intellectual dishonesty?

I'm not addressing the syllogism itself, but responding to the overall question by referring you to
Dr. Bo's:

Ambiguity Fallacy
(also known as: ambiguous assertion, amphiboly, amphibology, semantical ambiguity, vagueness)

Description: When an unclear phrase with multiple definitions is used within the argument; therefore, does not support the conclusion. Some will say single words count for the ambiguity fallacy, which is really a specific form of a fallacy known as equivocation.

Logical Form:

Claim X is made.

Y is concluded based on an ambiguous understanding of X.

Equivocation: a fallacy of ambiguity that sneakily changes a definition or sense of a keyword during a discussion.

This would be a simple non-sequitur as the conclusion does not follow.

Dr. Bo is right. The argument switches one word for a different word that means something else. "Cause" and "created" don't mean the same thing. To be valid, the syllogism must use the same words in the conclusion as the premise and must not switch their meanings. (That's where "ambiguity" comes in; see Walker's answer.

Also, the premise that everything that exists must have a cause is a metaphysical question, not a logical question. Note that at least some quantum physicists would probably argue that the premise is factually untrue. Here's one link:

https://phys.org/news/2015-02-big-quantum-equation-universe.html

And, yes, Jim, although you didn't use a bunch of fancy philosophical terms, your analysis is dead on target. Good work.