Via Hackernews I landed on "An Ancient Security Hole is (Not) Closed", which mentions "embargoes" on several occasions, e.g.:
...I have no interest in digging out more details or ruining embargoes that I'm not party to.
I know the generic meaning of "embargo", but am unsure of what the security-related version specifically means. Security.SE has no question on what such an embargo is. My Google-fu leads to only related articles or other types of embarboes.
Common sense tells me it's just that a target of a vulnerability would politely ask that you don't disclose them before they've fixed things. Is it just that, or is there more to it?