1974, introduced with the 15th edition of the Encyclopaedia Britannica, for the part of the encyclopedia in which information was presented in full essays, from macro- + ending from encyclopaedia.
Entries linking to macropaedia
macro-
word-forming element meaning "long, abnormally large, on a large scale," taken into English via French and Medieval Latin from Greek makros "long, large," from PIE root *mak- "long, thin."
encyclopaedia n.
see encyclopedia. The Latin spelling survives as a variant because many of the most prominent ones (such as Britannica) have Latin names.