Ligatures
In certain older texts (typically British), the use of the ligatures æ and œ is common in words such as archæology, diarrhœa, and encyclopædia. Such words have Latin or Greek origin. Nowadays, the ligatures have been generally replaced in British English by the separated digraph ae and oe (encyclopaedia, diarrhoea); but usually economy, ecology, and in American English by e (encyclopedia, diarrhea; but usually paean, amoeba, oedipal, Caesar). In some cases, usage may vary; for instance, both encyclopedia and encyclopaedia are current in the UK.
(See also: the section "ae and oe" in the article "American and British English spelling differences".)
Read more about this topic: English Orthography