5 grids (of which Texas is one), and they're not even just US ones - we share with Canada: https://i.cbc.ca/1.5921020.1613772812!/fileImage/httpImage/i...

They're then further subdivided into operators: https://en.wikipedia.org/wiki/North_American_power_transmiss...

In February of last year, a winter storm led to Texas having multi-day power outages, exasperated by the fact that ERCOT does not connect to any other grids.

According to https://en.wikipedia.org/wiki/Texas_Interconnection#Ties, the Texas grid has DC ties to the Eastern grid, and similar ties into Mexico.

Strictly speaking - 3 DC ties (from some cursory searching, they seem to only be used for power export) and 1 AC tie, that has only ever been used once. Texas is technically, but not meaningfully, connected to neighboring power grids.

I saw the term used to describe the various interconnected states but I think it's used because federal regulations can apply if one states connects with at least another, hence national