Deduction was a logical process in which an individual or group would gather data to support a line of reasoning, argument or theory. The Doctor utilised deduction on numerous occasions, describing the process of making deductions as "very exciting" in their eleventh incarnation. (TV: The Snowmen) The speed of logical deductions could be measured in gigaflops, at least when performed by a cybernetic being; K9 Mark 2 boasted of being capable of logical deduction "up to within 1.47593 gigaflops". (TV: Regeneration)
Behind the scenes
In the real world, contrary to the above, deduction is not actually about moving from observations to the most plausible conclusion. This, instead, is abduction. Deduction is generally about moving from general premises to more specific examples, and induction takes the reverse, from specific examples to more general statements. Abduction is specifically about plausible causal explanations for phenomena observed in the world. However, the sense of "deduction" corresponds to vernacular usage in the real world, notably in describing the "deductive abilities" of Sherlock Holmes and similar figures.