It’s unlikely you’ll ever write a letter to a Buddha. But if you did, how would you address her or him? “Dear Buddha” might do. But maybe it should be “Dear Your Holiness” or “Dear Venerated One” or something formal.
What follows is a traditional list of ten epithets, or titles, for a Buddha. These titles can be applied to all Buddhas, although sometimes in the context of a particular scriptural passage they might refer to a specific Buddha. Some of these titles are rarely used and may be unfamiliar to you. Others you are likely to bump into frequently.
For an example of how some of these terms might be used in scriptures, see the Saleyyaka Sutta of the Pali Sutta-pitaka (Majjhima Nikaya 41), which packs most of them into the second paragraph —
“And of that Master Gotama this fine reputation has spread: ‘He is indeed a Blessed One, worthy, & rightly self-awakened, consummate in knowledge & conduct, well-gone, a knower of the cosmos, an unexcelled trainer of those persons ready to be tamed, teacher of human & divine beings, awakened, blessed.”
Arhat (Sanskrit) or arahant (Pali) is a word that pre-dates Buddhism. In early Vedic texts it means “one who cannot be killed.” When used in Buddhist texts it is thought to mean “one who is worthy.”
The classic definition of an arhat is one who has attained enlightenment. Some teachers may quibble over the word “attained,” In Theravada Buddhism an arhat is distinguished from a Buddha because there can be only one Buddha per world age. Thus, a being who realizes enlightenment while there already is a Buddha is called arhat. However, a Buddha also is an arhat and may be addressed as such.
Bhagavan means “lord” or “master.” The Buddha is addressed as “Bhagavan” in many sutras. When paired with the word lokanatha it means “World Honored One,” a common title for the historical Buddha.
Lokavit (Sanskrit) or Lokavidu (Pali) refers to “one who understands the world” or “knower of the cosmos.”
This means “tamer of men,”and it refers to the ability of a Buddha to quiet fears and agitation.
6. Samyak Sambuddha
Some sources translate this title as “perfectly omniscient,” while others say it refers to a correctly (samyak) enlightened Buddha. In usage, it refers to one who has discovered the dharma teachings after they have disappeared from the world, and who has proclaimed those teachings to the world.
7. Sasta deva-manusyanam
This means “teacher of gods and men.” This term is found mostly in the Sutta-pitaka.
Sugata (Sanskrit) or sugato (Pali) means “well departed” or “gone to a good destination.”
Tathagata usually is explained as “the one who has thus come” or “the one who has thus gone.” This is one of the most common titles for a Buddha, and it can be found in both the Tipitaka and the Mahayana sutras. The historical Buddha referred to himself as Tathagata, and in some contexts the word might be referring to him. However, in other contexts it is referring to all Buddhas.
The precise meaning of the word is unclear. Many scholars relate it to tathata, which means “suchness” or “thusness” and refers to reality as it is, without illusions. In this interpretation, a tathagata would be one who has realized the truth about reality.
The word is also sometimes explained as meaning “one who is beyond coming or going.
Vidya-carana-sampanna (Sanskrit) or vijjā-caraṇa-sampanno (Pali) refers to one who has perfect knowledge and conduct.
[This is an article I wrote for the Buddhism section of About.com. However, since About.com has removed it from their servers, all rights revert to me.]