K.O.s ? GoBots with TF stickers slapped on LOL ? Re Issue Molded characters like G Max ? Micromasters like erector ? Jumpstarters ?
3rd Party TFs are Transformers made by 3rd party manufacturers/companies, ones that aren't related to Takara or Hasbro. A lot of them are original designed toys inspired by existing Transformers or add ons to existing Hasbro/Takara products, while some are KOs.
Third party Transformers: I'd say the Transformers community uses this term primarily for toys NOT made by either Hasbro or Takara, but which evidently and obviously lean on the intellectual property of the original Transformers toy brand as inspiration for newly designed toys or add-on parts. Knock offs are made by 'third party's', but are not usually considered true third party Transformers.
None of the above, though many people would argue that this is the closest. As in other threads, I recommend you GO HERE. This website is your friend, and will stop you getting flamed. Seriously, I've been into and collecting TF's since 1984 and I still find out stuff I don't know on the tfwiki. Third party is essentially a non-Transformer made by neither Hasbro or Takara, an unofficially transforming robot homage to a character you will probably recognize from Transformers mythology.
3rd party just refers to someone else outside of hasbro/takaratomy (1st party) making/distributing products to the consumer (2nd party). These can range from completely new molds, add ons, or ko's of existing product or designs. That all falls under 3rd party.