彻底(chè dǐ) - thorough, complete, thoroughly in Mandarin

彻底 (chè dǐ) means done all the way down, with nothing left half finished. Most of the time it sits in front of a verb as an adverb: 彻底解决, 彻底改变, 彻底忘记. Attach it to a noun with 的 and it becomes an adjective: 彻底的失败, 彻底的胜利.

It carries a note of finality that 完全 does not. 完全 is a neutral one hundred per cent (完全同意, completely agree), while 彻底 implies the job was taken to its root, often after a struggle.

The trap is using it for degree with an adjective. You cannot say 彻底高兴 the way English says utterly delighted; for that reach for 非常 or 极了. Keep 彻底 for actions and changes that were carried through to the end.