Scam is a fraud, plain and simple. In the crypto world, it's usually related to ICO's that are setup without any real plan to develop a project ... they just raise some funds and then vanish.
Probably the most known crypto scam to date was Bitconnect ... supported by many youtubers and influencers in the crypto space, bearing the signs of a typical ponzi referral system.
The biggest ICO sam to date is probably the Pincoin and iFan fiasco, organized by the same company out of Vietnam ... leaving about 32 000 investors with a combined loss of $660 mln.