What is terminal on mac
It's rather complicated, and actually a lot of the complexity is to avoid wasting space; I don't think you can "reclaim" anything without breaking it.
Let me start at the beginning: your hard drive (/dev/disk0) has two relevant partitions: Macintosh HD (your regular startup volume), and Recovery HD.
Recovery HD is marked in the partition table with the type Apple_Boot, but is actually in the normal HFS+ format. It contains minimal booter files and kernel, and at /com.apple.recovery.boot/BaseSystem.dmg, a disk image with a stripped-down and tweaked copy of OS X. The booter mounts this volume (it attaches as /dev/disk1), and transfers to OS X running on it. This is the Mac OS X Base System.
Notice that the Recovery HD is only 650MB, but Mac OS X Base System is 1.4GB? That's because it's a compressed disk image (and I'm pretty sure that compression is the reason they bother with all this disk image
trickery). Actually, BaseSystem.dmg is compressed down to only 451MB (at least in OS X v10.7.0).
Also, the volume naming is somewhat inconsistent. You've got /dev/disk1s3 named "Recovery HD", but for some reason it's mounted as "/Volumes/Image Volume" in recovery mode. BaseSystem.dmg has a volume named "Mac OS X Base System".
So that's disk0 and disk1; what about the rest? I'm not certain, but I'm pretty sure they are RAM disks to save temporary data in folders OS X modifies as it runs (remember that in recovery mode, you're running from a read-only disk image). Running the mount command in recovery mode is informative:
Those "union" attributes mean that things in the relevant folder in the startup volume will be visible, but anything modified gets stored in what I'm pretty sure is a RAM disk.
If you want to look at this stuff yourself, you can mount the relevant volumes from the regular OS:Source: apple.stackexchange.com