What is "neutral surface"

Neutral \Neu"tral\, a. [L. neutralis, fr. neuter. See Neuter.]

1. Not engaged on either side; not taking part with or assisting either of two or more contending parties; neuter; indifferent.

   The heart can not possibly remain neutral, but constantly takes part one way or the other.
   --Shaftesbury.

2. Neither good nor bad; of medium quality; middling; not decided or pronounced.

   Some things good, and some things ill, do seem, And neutral some, in her fantastic eye.
   --Sir J. Davies.

3. (Biol.) Neuter. See Neuter, a., 3.

4. (Chem.) Having neither acid nor basic properties; unable to turn red litmus blue or blue litmus red; -- said of certain salts or other compounds. Contrasted with acid, and alkaline.

   Neutral axis, Neutral surface (Mech.), that line or plane, in a beam under transverse pressure, at which the fibers are neither stretched nor compressed, or where the longitudinal stress is zero. See Axis.

   Neutral equilibrium (Mech.), the kind of equilibrium of a body so placed that when moved slighty it neither tends to return to its former position not depart more widely from it, as a perfect sphere or cylinder on a horizontal plane.

   Neutral salt (Chem.), a salt formed by the complete replacement of the hydrogen in an acid or base; in the former case by a positive or basic, in the latter by a negative or acid, element or radical.

   Neutral tint, a bluish gray pigment, used in water colors, made by mixing indigo or other blue some warm color. the shades vary greatly.

   Neutral vowel, the vowel element having an obscure and indefinite quality, such as is commonly taken by the vowel in many unaccented syllables. It is regarded by some as identical with the [u^] in up, and is called also the natural vowel, as unformed by art and effort; it is also called the indefinite vowel. It is symbolized in some phonetic alphabets by the schwa ([schwa]). See Guide to Pronunciation, [sect] 17.