No-see-um screen
Also called: 20x20 mesh, no-see-um mesh, 20x20 screen.
No-see-um screen is 20x20 pool-cage mesh: the same fiberglass material as standard 18x14 screen, woven tighter to stop the no-see-ums, gnats and sand flies that walk right through 18x14. The numbers are the mesh count, strands per inch each way.
Why it matters
It is an airflow-and-view versus tiny-bug trade. Standard 18x14 gives the best breeze and the clearest view and is the cheapest. 20x20 costs modestly more, cuts airflow and visibility slightly, and adds a bit of daytime privacy. Its Florida lifespan is similar to standard fiberglass, about 5–7 years. Choose it if you are coastal, near marsh or mangroves, or getting bitten by things you cannot see; inland and bug-tolerant, 18x14 is fine. In a mixed cage it goes on the big upper panels. If you are being bitten by things you cannot see, that is a rescreen decision, not a spray decision.
In numbers
18x14 means 18 strands per inch one way and 14 the other. 20x20 is the tighter weave, and that mesh count, not the roll width the installer buys, is the real decision.