What happens if I stop defragmentation?

I'm using Disk Defragmenter to defragment a volume (almost 160 GiB in size). I thought it would stop after 100% defragmentation, but now I see there are multiple "passes" for "consolidation". The temperature of my hard drive has already reached 68 °C, so I don't really feel comfortable continuing. What would happen if I stopped the operation? Is it trivial to continue or would it make things worse?

You can safely stop Disk Defragmenter, so long as you do it by clicking the Stop button, and not by killing it with Task Manager or otherwise "pulling the plug." Disk Defragmenter will simply complete the block move it is currently performing, and stop the defragmentation.