John R. Ragazzini is credited with coining the term  "Operational Amplifier ". 

The June 2005 Vol25 No3 issue of  the IEEE's Control Systems Magazine has an excellent retrospective and current state of the Analog Computer. Dr. Kent Lundberg of MIT, edited the 9 articles on this topic in this issue. On page 67 of this issue there is the following excerpt:

 "As an amplifier so connected can perform the mathematical operations of arithmetic and calculus on the voltages applied to it's input, it is hereafter termed an  "Operational Amplifier ".

-John R. Ragazzini et al.,  "Analysis of problems in dynamics by electronic circuits", Proceedings of the IRE, vol. 35. p. 444, May1947. "

The following scans of this Ragazzini article came from the famous Palimpsest mentioned in the Recommended Reading section.