厕所 (cèsuǒ) is the most direct and common word for 'toilet' or 'restroom' in everyday Chinese. It's more blunt than the polite euphemisms 洗手间 (xǐshǒujiān, 'washroom') and 卫生间 (wèishēngjiān, 'bathroom'), which are preferred in public places and homes. Use 厕所 when you need to be clear and direct; use 洗手间 or 卫生间 to sound more polite.