I'll keep this short since most of you probably know the difference, but I'm still gonna post this for noobies.
Lost, by definition, Is: Unable to find one's way; not knowing one's whereabouts. I put the important part in bold.
Undumped, by definition, is: A video game that has not been dumped and uploaded to the internet.
Examples of lost games: Taiwan 2001 (spiritual successor to Hong Kong 97), Bouncer, Circus Charlie (Atari 2600), and Sonic the Hedgehog 1990 Tokyo Toy Show Prototype.
Examples of undumped games: Mortal Kombat: Nitro Edition (Dev has the prototype cart, but hasn't dumped it as of 7/2/20), Western Gun, and Grand Champion.
If you have any questions don't hesitate to ask.