Learn French: Le confinement est long, voire interminable. - The lockdown is long, and even endless.

Voire doesn't just link two ideas; it introduces an escalation or a stronger alternative. While et just means "and", voire means "and even" or "if not". It shows that the lockdown isn't just long, it's pushing into the territory of being endless (interminable).

Is voire related to the verb voir (to see)?

Despite the spelling, no! Voire comes from an old French word meaning "truly" (related to the Latin "vera"). It has nothing to do with seeing, so don't try to conjugate it or confuse it with the verb voir.

I often hear French people say voire même. Could we say long, voire même interminable here?

Yes, in everyday spoken French, you will hear voire même very often. However, strict grammarians consider it a redundancy, because voire already contains the idea of même (even). In a formal writing exam at the B2 level, it is safer to stick to just voire, as written in this sentence.

Is le confinement only used to talk about pandemic lockdowns?