Persian Empire

Collocation of 'Persian' (as a demonym of Latinate Persia in the sense of Greek Persis, Old Persian Pārsa) and 'empire'. Translates Latin Imperium Persārum, as attested e.g. in the 1st century (e.g. Quintus Curtius Rufus "Life and exploits of Alexander the Great" 3.3.8).
In English from the 17th centuryhttp://books.google.com/books?id=lLhHAAAAIAAJ&pg=PA37&dq=%22Persian+Empire%22&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=1700&as_brr=0#v=onepage&q=%22Persian%20Empire%22&f=falsehttp://books.google.com/books?id=cY8pAAAAYAAJ&pg=RA3-PA400&dq=%22Persian+Empire%22&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=1800&as_brr=0#v=onepage&q=%22Persian%20Empire%22&f=false, invariably in reference to the Achaemenid Empire (550–330 B.C.E.) conquered by Alexander the Great. Sometimes expanded to Medo-Persian Empirehttp://books.google.com/books?id=AOYqAAAAYAAJ&pg=PA33&dq=%22Medo-Persian+Empire%22&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=1800&as_brr=0#v=onepage&q=%22Medo-Persian%20Empire%22&f=false (translating Imperium Persārum et Mēdōrum, "empire of (ruled by) Persians and Medians"). The 'et Medorum' is not a reference to Median empire (625–550 B.C.E.), but rather to (1) the tribal affiliation and shared status of the Persians and Medians, (2) the mixed Persian-Median parentage of the founder of the dynasty, Cyrus I.
The term 'Persian empire' has sometimes been extended to the later, "restored" empire ruled by the Sasanian dynastyhttp://books.google.com/books?id=YfoEAAAAQAAJ&pg=PA129&dq=%22Persian+Empire%22+Sassanian&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=1850&as_brr=0#v=onepage&q=%22Persian%20Empire%22%20Sassanian&f=falsehttp://books.google.com/books?id=PNYLJakw1SQC&pg=PA28&dq=%22Persian+Empire%22+Sassanian&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=1980&as_maxm_is=0&as_maxy_is=&as_brr=0#v=onepage&q=%22Persian%20Empire%22%20Sassanian&f=falsehttp://books.google.com/books?id=MziRd4ddZz4C&pg=PA100&dq=%22Persian+Empire%22+Sassanian&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=1980&as_maxm_is=0&as_maxy_is=&as_brr=0#v=onepage&q=%22Persian%20Empire%22%20Sassanian&f=false, the ruling dynasty of which was also from Persia in the sense of Greek Persis, Old Persian Pārsa. It is sometimes taken more loosely to refer to the Achaemenid, Parthian and Sasanian states taken together (so Encyclopædia Britannica 2009: "historical empire from about 550 B.C.E. – 640 C.E.").