A (potentially distributed) datastructure of some kind, which resembles a linked-list, and ensures its integrity using some sort of cryptographic hash of the previous link such that changing any link but the most recent invalidates the whole chain. Literally -- a chain of (data) blocks.
A non-engineering description of a "blockchain" should be considered to be misleading.
edit - also, I'd take cryptocurrencies more seriously if they could stay out of the headlines for a while
Is utterly funny to read people's opinions bashing against blockchain technology and when asked about a simple definition they fail to mention the use of consensus protocols which was, already 10 years ago, the huge innovation that lead to this revolution.
I agree that a token based currency is not required, I'm just interested in what people define a blockchain as for intellectual curiosity.