阁下为第二人称的敬称或提称语，与「足下」意思相近。初为对显贵者的尊称，后乃泛用。也作「合下」。古代常见于口语，近代至现代则多用于书籍信函中。目前人们在书信（尤其是公函、业务相关书信）中仍然使用阁下来称呼彼此。但是由于“您”一字的代替，慢慢地“阁下”（英语：Excellency）一词多用于外交场合。

阁下亦作为对身分特殊的人之尊衔，用以尊称某些显赫尊贵、有地位的人士，与陛下、殿下相类，但比较普及，使用泛范较广泛，并且比尊称君王的陛下、殿下次级一些。在较现代化的场合，一般都使用“阁下”一词，代替其他古代的尊衔。

Excellency is an honorific style given to certain members of an organisation or state. Once entitled to the title "Excellency", the holder usually retains the right to that courtesy throughout his or her lifetime, although in some cases the title is attached to a particular office, and is held only for the duration of that office.