Saturation is the point at which a format, a hook or a topic has been shown to most of the audience that was ever going to respond to it. Performance starts declining not because the content got worse, but because fewer people are seeing it for the first time.
The word is borrowed from market saturation, the same idea one level up: a market reaches the point where most of the people who would want something already have it, or have already been offered it, and growth has to come from somewhere new.
A saturated format is not necessarily a bad one; it means the easy gains from posting it again have already been taken, and the next post has to bring something the earlier ones did not.
Recognising saturation early is mostly a matter of watching a format's own numbers decline over several posts rather than one, since a single weak result is ordinary variation and a consistent slide across a run of them is the format telling an account something.
A common mistake is confusing saturation with a format simply going stale creatively — a well-made video in a saturated format can still underperform for reasons that have nothing to do with quality, because the pool of people who had not already seen the idea has shrunk.
Saturation is also read against a competitor set, not an account in isolation: a topic an account has posted only twice can already be saturated if several competitors have covered the same angle repeatedly, since the audience that would respond to it has already been reached by someone else's version.
Where it comes from
G2's technology glossary describes market saturation in close to these terms: a market reaching its maximum potential, with no significant new customers left to reach. The exact wording on the live page was not independently re-confirmed for this entry, so it is paraphrased rather than quoted.
Read the source: G2 — Market Saturation.
What Rankfield reads
The landing page answers this directly, in its own words:
Your account end to end, your competitors, and the wider market around you: posts, videos, captions, comments, rhythm, what you offer. It watches the videos, not just the numbers under them.