Compute Engine virtual machines that use Google's spare capacity at up to a 91% discount. Google can interrupt them with 30 seconds' notice when it needs the capacity back. They replace the older preemptible VMs, which lasted at most 24 hours.
In CDL they appear in cost-optimization questions. Question: batch jobs, rendering, or ML training that tolerate interruptions and use checkpoints → Spot VMs. They are not suitable for web servers or databases that must always be available.
Spot VMs is a concept that can show up on the exam. Check the full guide to see it in context.