In _The Big Three in Economics_, Mark Skousen says in footnote 2 on p. 137:

With Keynes came the division of "macroeconomics," the study of 
economic aggregates such as the price level, the money supply, and 
Gross Domestic Product, and "microeconomics," the theory of 
individual and firm behavior.  Paul Samuelson, who did not use the 
term in the first edition of his textbook, _Economics_ (1948), says 
the distinction between "micro" and "macro" goes back to 
econometricians Ragnar Frisch and Jan Tinbergen, the first Nobel 
Prize winners in economics. But Roger Garrison notes that the 
Austrian economist Eugen Bohm-Bawerk wrote this sentence in January, 
1891: "One cannot eschew studying the microcosm if one wants to 
understand properly the macrocosm of a developed country" 
(Bohm-Bawerk 1962: 117).

Humberto Barreto