A thorough, major cleaning (e.g., year-end cleaning). Used as a noun, and commonly as a 'suru' verb (大掃除する).
年末に家族みんなで大掃除をするのが習慣だ。
It's a custom for the whole family to do a big cleaning at the end of the year.
引っ越し前にアパートの大掃除をしておいた。
I gave the apartment a thorough cleaning before moving out.
大掃除のために掃除道具を買い揃えた。
I bought cleaning supplies for the big cleanup.