It's a misnomer, but is someone's attempt to define any tech company which is so far apart from the rest of the pack in terms of business practices and reach that they don't warrant being called 'yet another tech company'.

This usually involves under handedness, tax loopholes, and secret handshakes that 'normal' startups are too scared to execute on. 'Normal' startups have a market safety net to fall back on and can pivit anytime they want without much fanfare caused. Unicorns are not afforded this luxury because they are, well, unicorns.