1.
time; the passage of time
A literary word for time, especially the swift passage of time. Most commonly encountered in the proverb 光陰矢の如し (time flies like an arrow).
Usage
A literary and somewhat classical word. In everyday speech, 時間 or 時 is used instead. 光陰 appears most often in proverbs, essays, and formal writing to evoke a poetic sense of time passing.
Etymology
From Chinese. 光 (light) refers to sunlight (daytime) and 陰 (shadow/dark) refers to the moon (nighttime), together representing the cycle of days and the passage of time.