A micronation, also known as an ephemeral state or project nation, is a self-declared entity that declares independence yet lacks recognition from other nations wor the UN.
Microstate: a self-declared entity that does have recognition from other nations or the UN. Examples include Monaco and Liechenstein.
Unrecognised state: an entity that has a government and controls territory but lacks recognition from other nations or the UN. The main difference lies in control - a micronation doesn't control the territory it claims whereas an unrecognised state does. Examples include Taiwan and Kosovo.
Anybody can start a micronation! Micronations have no legal standing, so you don't need governmental permission or approval to start one.
If you're interested, check out our How do I start a micronation? article!