Hi Graham. Your definition can be in the right way but maybe is not “friendly” for generalist audience. As I wrote, in a previous post I proposed the following definition:

A blockchain is a set of techniques, protocols and tools that allow to have a non-modifiable digital archive of transactions, distributed among the nodes of a network and periodically validated by them.

Another example. In “Blockchain Technology Overview” draft of National Institute of Standards and Technology (NIST) there is this (long) definition

Blockchains are immutable digital ledger systems implemented in a distributed fashion (i.e.,without a central repository) and usually without a central authority. At their most basic level,they enable a community of users to record transactions in a ledger that is public to that community, such that no transaction can be changed once published.

I hope these informations can help you. Good luck for Lecture!