You can have a one atom molecule, depending upon which definition you look at.

I have never seen that definition and I can't find it online anywhere. Wikipedia gives 6 pretty solid looking references for the "two or more atoms" definition.

The same Wikipedia page on molecules that says they have "two or more atoms held together" also says this in the next paragraph:

In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules despite being composed of a single non-bonded atom.

Well, my bad, should have read farther.

There isn't a single definition of "molecule" that includes fewer than two atoms.

One google search and I saw two of them, including in Wikipedia.