Tank usually refers to a character's durability in battle like if someone gets hit by some type of attack and survives it with minimal injury. 

Some good examples would be:

- Hellmaster Phibrizzo from Slayers was able to survive a universe devastating blast from Lina Inverse.
- Ushio from Ushio and Tora tanking a blast from Hakumen no Mono.
- Mu Alexius from Magi tanking a blast of Titus Alexius.
- Shuda from RAVE tanking Megido's heat attack.
- Toguro fromYu Yu Hakusho tanking Yusuke Urameshi's reigan.