洗澡 (xǐ zǎo) is the everyday word for taking a shower or bath — a specific activity, unlike 洗 (xǐ) which is the general verb 'to wash' (hands, dishes, etc.). It is a separable verb (离合词): you can insert words like 了 or a measure word (e.g., 洗了一个澡). 沐浴 (mù yù) is a formal/literary term, not used in daily speech.