A "computational knowledge engine that draws on multiple sources to answer user queries directly". A search within WolframAlpha generates output by doing computations from its own internal knowledge base. Some examples include: mathematics, statistical probabilities, conversions and calculations (electric circuit calculations), physics, and many more.
As a fun example enter 17 miles per gallon to metric to see how imperial can be easily converted.
The ACS Style Guide: Effective Communication of Scientific Information (3rd ed, 2006)
Online version is accessible with ACS membership