From Wikipedia: A spike is a product development method originating from Extreme Programming that uses the simplest possible program to explore potential solutions. It is used to determine how much work will be required to solve or work around a software issue.

“In XP, we don’t divide and conquer. We conquer and divide. First we make something that works, then we bust that up and and solve the little parts.”
Kent Beck

A spike solution is a technical prototype that drives a spike through all the components / layers of the proposed solution first before scaling up.