What is the formal definition of informatics?
The Supplement to the Oxford English Dictionary (1976) defines informatics as "the discipline of science which investigates the structure and properties (not specific context) of scientific information, as well as the regularities of scientific information activity, its theory, history, methodology and organization. The Random House dictionary (1987) is a bit less rigid and defines it as the study of information processing; computer science.
A more proper definition may lie between the two extremes, and we could say that informatics is the study and application of information technology.
Pharmacy Informatics then, would be the application of this discipline to the art and practice of pharmacy
For more information on the origins and meaning of the term, the reader may want to peruse A history of Medical Informatics in the United States (1950-1990) by Morris Frank Collen, published by the American Medical Informatics Association.