Definition of a "lexicon" in Named Entity Recognition
As far as I know the word "lexicon" is mostly used for a simple list of words (or terms), I would say it's quite rare to use it for describing a list of patterns/rules. "gazetteer" and "dictionary" would be a bit more general in my opinion, for instance one can have a "dictionary of rules" which associates specific patterns with actions. But overall I agree with your impression that these terms are often used interchangeably, I'm not aware of any standard definition.