The word "nominal" has a number of definitions.
For example, the Free Dictionary gives seven:
nom·i·nal (nm-nl) adj.
- a. Of, resembling, relating to, or consisting of a name or names. b. Assigned to or bearing a person's name: nominal shares.
- Existing in name only.
- Philosophy Of or relating to nominalism.
- Insignificantly small; trifling: a nominal sum.
- Business a. Of, relating to, or being the amount or face value of a sum of money or a stock certificate, for example, and not the purchasing power or market value. b. Of, relating to, or being the rate of interest or return without adjustment for compounding or inflation.
- Grammar Of or relating to a noun or word group that functions as a noun.
- Aerospace & Engineering According to plan or design: a nominal flight check. [...]
[Middle English nominalle, of nouns, from Latin nōminālis, of names, from nōmen, nōmin-, name; see nō̆men- in Indo-European roots.]
This consistent with the definitions in OED3 and other sources.
Most of the definitions are clearly from the sense of "names" and "nouns" - and from that idea of something being only in name, but not in reality.
However, the aerospace sense seems quite different. During a recent rocket launch, the announcer repeated phrases like "Height is nominal. Power is nominal." to mean these values were within the acceptable and expected ranges.
The OED references these sources [Hat tip @tchrist]:
6. (See quot. 1970.)
1966 Aviation Week & Space Technology 5 Dec. 30/1
The mission is to launch the 800-lb. Prime vehicle to effect a nominal re-entry at 400,000 ft. following injection at 26,000 fps.
1970 N. Armstrong et al. First on Moon vi. 124
An example of misuse is our use of the word ‘nominal’, which most of the English-speaking world interprets as meaning small, minimal-and we usually use it in the sense of being average or normal.
1970 R. Turnill Lang. Space 94
Nominal, a favourite word, meaning within prescribed limits; anything from ‘perfect’ to acceptable.
1972 Daily Colonist (Victoria, B.C.) 26 July 3/1
As one engineer said, ‘She is phenomenally nominal’ — nominal being space jargon for operating-as-planned.
I wonder if anyone can explain that derivation. Is it derived from the other senses of nominal? Is it influenced by the word "normal"?