The Collaborative International Dictionary of English v.0.48:

Mid \Mid\ (m[i^]d), a. [Compar. wanting; superl. Midmost.]
   [AS. midd; akin to OS. middi, D. mid (in comp.), OHG. mitti,
   Icel. mi[eth]r, Goth. midjis, L. medius, Gr. me`sos, Skr.
   madhya. [root]271. Cf. Amid, Middle, Midst, Mean,
   Mediate, Meridian, Mizzen, Moiety.]
   [1913 Webster]
   1. Denoting the middle part; as, in mid ocean.
      [1913 Webster]

            No more the mounting larks, while Daphne sings,
            Shall list'ning in mid air suspend their wings.
                                                  --Pope.
      [1913 Webster]

   2. Occupying a middle position; middle; as, the mid finger;
      the mid hour of night.
      [1913 Webster]

   3. (Phon.) Made with a somewhat elevated position of some
      certain part of the tongue, in relation to the palate;
      midway between the high and the low; -- said of certain
      vowel sounds; as, [=a] ([=a]le), [e^] ([e^]ll), [=o]
      ([=o]ld). See Guide to Pronunciation, [sect][sect] 10, 11.
      [1913 Webster]

   Note: Mid is much used as a prefix, or combining form,
         denoting the middle or middle part of a thing; as,
         mid-air, mid-channel, mid-age, midday, midland, etc.
         Also, specifically, in geometry, to denote a circle
         inscribed in a triangle (a midcircle), or relation to
         such a circle; as, mid-center, midradius.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Midmost \Mid"most`\, a. [OE. middemiste. Cf. Foremost.]
   Middle; middlemost.
   [1913 Webster]

         Ere night's midmost, stillest hour was past. --Byron.
   [1913 Webster]

WordNet (r) 3.0 (2006):

midmost
    adv 1: the middle or central part or point; "in the midst of the
           forest"; "could he walk out in the midst of his piece?"
           [syn: midmost, in the midst]
    adj 1: being in the exact middle [syn: middlemost, midmost]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "midmost":
   amidships, average, axial, center, centermost, central,
   centralized, centric, centroidal, centrosymmetric, concentrated,
   core, diameter, diaphragm, equator, equatorial, equidistant,
   geocentric, halfway, heart, interior, intermediary, intermediate,
   kernel, key, mean, medial, median, mediocre, mediterranean, medium,
   mesial, mezzo, mid, middle, middlemost, middling, midland, midriff,
   midships, midst, midway, nuclear, nucleus, omphalic, pivotal,
   thick, thick of things, umbilical, waist, waistline, zone