• Webster's Revised Unabridged Dictionary
    • Dimidiate (Biol) Consisting of only one half of what the normal condition requires; having the appearance of lacking one half; as, a dimidiate leaf, which has only one side developed.
    • Dimidiate Divided into two equal parts; reduced to half in shape or form.
    • Dimidiate (Biol) Having the organs of one side, or half, different in function from the corresponding organs on the other side; as, dimidiate hermaphroditism.
    • Dimidiate To divide into two equal parts.
    • Dimidiate (Her) To represent the half of; to halve.
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Century Dictionary and Cyclopedia
    • dimidiate To divide into two equal parts. In her.: To cut in halves, showing only one half. Thus, when a shield bearing a lion is impaled with a shield bearing a chevron, these bearings may be each represented in full in the half shield, or each bearing may be dimidiated—that is, one half of the lion and one half of the chevron only shown. This, however, is liable to lead to confusion, and is rare.
    • dimidiate Divided into two equal parts; halved; hence, half the usual size, or half as large as something else. Specifically— In botany and entomology, having, as an organ, one part so much smaller than the other as to appear to be missing, or altogether wanting.
    • dimidiate In heraldry, reduced or diminished by half.
    • ***
Chambers's Twentieth Century Dictionary
    • adj Dimidiate di-mid′i-āt divided into halves: having a shape that appears as if halved
    • v.t Dimidiate (her.) to represent the half of
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Webster's Revised Unabridged Dictionary
L. dimidiatus, p. p. of dimidiare, to halve, fr. dimidius, half. See Demi-


In literature:

The shape of the fungus is peculiar, a sort of semi-circular outline that may be called dimidiate.
"Among the Mushrooms" by Ellen M. Dallas and Caroline A. Burgin
We have accounts of dimidiate hermaphrodite lobster, male in one half and female in the other half of the body.
"The People's Common Sense Medical Adviser in Plain English" by R. V. Pierce
Harcourt, two bars, is dimidiated, and meets Beke, a cross moline or ancree.
"Notes and Queries, Number 191, June 25, 1853" by Various
Dimidiate -us: halved; extending half way around; applied to elytra when they cover only half the abdomen.
"Explanation of Terms Used in Entomology" by John. B. Smith
Dimidiation, accollation, and impalement succeeded each other at short intervals.
"Notes and Queries, Number 210, November 5, 1853" by Various
The correct blazon, I believe, would be: Or, an eagle double-headed, displayed sable, dimidiated, and impaling gu.
"Notes and Queries, Number 223, February 4, 1854" by Various
LA TOUCHE, J. D., on a Canadian apple with dimidiate fruit, i.
"The Variation of Animals and Plants Under Domestication, Volume II (of 2)" by Charles Darwin
The pileus is corky, dimidiate, sessile.
"The Mushroom, Edible and Otherwise" by M. E. Hard
P. tough, thin, unequal, excentric, dimidiate, cinnamon then pale, becoming squamulose, 3-9 cm.
"European Fungus Flora: Agaricaceae" by George Massee
And the chief standard of the people, which was the captain's, was dimidiated white and red.
"Villani's Chronicle" by Giovanni Villani
That this singular shield is a dimidiation of two antient coats cannot be doubted.
"The Curiosities of Heraldry" by Mark Antony Lower
Very slender, innovate-branching; leaves smaller, especially above, dimidiate-ovate or subfalcate, subdecurrent.
"The Manual of the Botany of the Northern United States" by Asa Gray
This "dimidiation," however, had its inconvenience.
"Encyclopaedia Britannica, 11th Edition, Volume 13, Slice 3" by Various