疲劳 is the formal, often written word for 'fatigued, weary', emphasizing physical or mental exhaustion from sustained effort. It is stronger in tone than the everyday spoken word 累 (lèi, tired) and more clinical than 疲倦 (píjuàn, tired/weary). You'll see 疲劳 in medical, academic, or formal contexts, such as 疲劳驾驶 (fatigued driving).