What is the difference between archaic and obsolete words?

The meaning of these temporal labels can be somewhat different among dictionaries and thesauri. The label archaic is used for words that were once common but are now rare. Archaic implies having the character or characteristics of a much earlier time. Obsolete indicates that a term is no longer in active use, except, for example, in literary quotation. Obsolete may apply to a word regarded as no longer acceptable or useful even though it is still in existence.

In Dictionary.com, the archaic label is described this way: “Archaic is used as a label in this dictionary for terms and definitions that were current roughly as late as 1900 but are now employed only as conscious archaisms.” It describes the obsolete label thus: “Terms and definitions labeled Obsolete in this dictionary have not been in widespread use since the mid 1700s. Unlike some relatively familiar archaic words and phrases, like prithee and thou art, obsolete words and phrases are not easily understood by a modern reader, and obsolete senses of current terms.”