"Spike" comes from Extreme Programming where it means specifically code written without TDD. Code that wasn't written against a pre-existing test is by definition broken, and should not go into production. So you create a simple "spike solution" to explore the problem space, so called because it cross-cuts concerns like a spike going through wood or something, I dunno.

Spikes are explicitly time-boxed to keep the team focused on the red-green-refactor churn which is the software engineering process in XP.