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.
