Skills gap
The difference between what a posting asks for and what you can currently demonstrate — most of which is smaller than it looks.
Requirement lists are aspirational. The widely cited pattern is that applicants who meet most but not all of a list are frequently hired, because the list was assembled from several people's wishes and the hiring manager knows which two items are load-bearing. Self-rejecting on a list you meet two-thirds of is the most expensive habit in a job search.
The gaps that actually block you are the ones the job cannot be done without, and they're usually few: a specific certification or license, the core language or tool of the daily work, domain knowledge in a regulated field. Everything else is trainable and the employer knows it.
The useful move is to identify which of your gaps is real and address it directly rather than hiding it — naming a gap alongside the nearest adjacent thing you have done is more persuasive than hoping nobody asks. It also tells you where a few weeks of deliberate learning would change your options.
Job posting
The employer's public description of a role — part specification, part marketing, and rarely a literal list of requirements.
Keyword optimization
Using the employer's own vocabulary for skills and tools you genuinely have, so you appear in the searches a recruiter runs.
Match score
A rating of how well one candidate profile fits one job posting, used to rank a long list of openings before a human reads any of them.