'乏力' (fá lì) literally combines '乏' (fá), meaning 'lacking' or 'insufficient', and '力' (lì), meaning 'strength' or 'power'. Together, they form an adjective describing a physical state of weakness, exhaustion, or lack of energy — not merely tiredness, but a deeper, often medically noted absence of muscular or vital strength. It frequently appears in clinical contexts, such as symptom reporting or medical diagnoses.

The term carries a neutral-to-formal register and is commonly used by patients describing how they feel ('我最近总是乏力'), doctors documenting signs ('患者主诉乏力两周'), or health-related media discussing conditions like anemia, chronic fatigue, or post-illness recovery. Unlike colloquial terms like '累' (lèi, 'tired'), '乏力' emphasizes objective bodily incapacity rather than subjective weariness — it implies one cannot exert effort even if motivated.