1Extermination or elimination of pests, harmful things, or problems; getting rid of or suppressing something.
台所に出たゴキブリを退治した。
I exterminated a cockroach that appeared in the kitchen.
害虫退治のために農薬を散布した。
They applied pesticide to control pests.
問題を早めに退治しておかないと大きくなる。
If you don't nip the problem in the bud, it will grow bigger.
2In Buddhism: renouncing worldly desires and attachments; removing defilements to concentrate on teachings or practice.
修行者は欲望を退治することを目指すと言われる。
Practitioners are said to aim at eliminating their desires.
3Curing an illness; getting rid of a disease or symptoms (colloquially 'to beat' an illness).
風邪はできるだけ早く退治したほうがいい。
It's best to get rid of a cold as quickly as possible.
医師の治療で感染症を退治できた。
The infection was eradicated thanks to the doctor's treatment.