• WordNet 3.6
    • n trefoil an architectural ornament in the form of three arcs arranged in a circle
    • n trefoil a plant of the genus Trifolium
    • n trefoil any of several Old World herbs of the genus Medicago having small flowers and trifoliate compound leaves
    • ***
Webster's Revised Unabridged Dictionary
    • Trefoil (Her) A charge representing the clover leaf.
    • Trefoil (Arch) An ornamental foliation consisting of three divisions, or foils.
    • Trefoil (Bot) Any plant of the genus Trifolium, which includes the white clover, red clover, etc.; -- less properly, applied also to the nonesuch, or black medic. See Clover, and Medic.
    • ***
Century Dictionary and Cyclopedia
    • n trefoil In anatomy, the triangular area which forms the front part of a molar tooth; the protocone, paracone, and metacone lying at the angles. Same as trigon, 6.
    • n trefoil A plant of the genus Trifolium; clover. The name is given to various other plants with trifoliolate leaves, in England somewhat specifically to the black medic, Medicago lupulina, grown for pasture. See clover, Stylosanthes, and specific names below.
    • n trefoil The third leaf put forth by a young plant.
    • n trefoil An ornamental feathering or foliation used in medieval Pointed architecture in the heads of window-lights, tracery, panelings, etc., in which the spaces between the cusps represent a three-lobed figure.
    • n trefoil In heraldry, a bearing supposed to represent a clover-leaf. It consists usually of three rounded and slightly pointed leaves set in a formal way at the three upper extremities of a small cross, the lower extremity of which terminates in different ways. Also trefle.
    • n trefoil A bombycid moth, Lasiocampa trifolii, whose larva feeds on grass and clover in Europe. Also called grass-egger and clover-egger.
    • trefoil Characterized by the presence or prominence of a trefoil or trefoils; consisting of trefoils; thrice foliated.
    • ***
Chambers's Twentieth Century Dictionary
    • n Trefoil trē′foil a three-leaved plant, as the white and red clover:
    • n Trefoil trē′foil (archit.) an ornament like trefoil
    • ***


Webster's Revised Unabridged Dictionary
L. tres, three + E. foil, leaf; cf. F. frèfle, It. trifoglio, L. trifolium,. See Tri- Foil leaf, and cf. Trifoly
Chambers's Twentieth Century Dictionary
L. trifoliumtres, three, folium, a leaf.


In literature:

There is a trefoiled arch, the cusps having circular terminations with the star ornament.
"Bell's Cathedrals: The Cathedral Church of Ely" by W. D. Sweeting
Red Birdsfoot Trefoil, 559.
"The Field and Garden Vegetables of America" by Fearing Burr
The tympanum of the large arch is pierced with a quatrefoil or trefoil.
"Scottish Cathedrals and Abbeys" by Dugald Butler and Herbert Story
The arch over it is a moulded trefoil arch, surmounted by a plain canopy of very simple and formal design.
"Bell's Cathedrals: The Abbey Church of Tewkesbury" by H. J. L. J. Massé
Its inner trefoil arch is of a form very uncommon in this country, but more usual on the Continent.
"Bell's Cathedrals: The Cathedral Church of Rochester" by G. H. Palmer
Those that flank the portal have each a large niche at the bottom, with engaged shafts, and the head and dripstone trefoiled.
"Bell's Cathedrals: The Cathedral Church of Ripon" by Cecil Walter Charles Hallett
It is much rarer to find the wood-sorrel trefoil with a fourth leaflet than it is to find the clover trefoil so provided.
"More Science From an Easy Chair" by Sir E. Ray (Edwin Ray) Lankester
Do you wish for some more trefoil leaves?
"Fairy Tales of Hans Christian Andersen" by Hans Christian Andersen
Trefoils of carved wood adorned the upper portions of the doors.
"Sentimental Education, Volume II" by Gustave Flaubert
Four white trefoils sprang up where she trod, and therefore was she called Olwen.
"The Lilac Fairy Book" by Andrew Lang

In poetry:

across the avenue a trefoil lamp
of the streets tosses luckily.
The leaves, finally, love us! and
moonrise! we die upon the sun.
"Ann Arbor Variations" by Frank O Hara
Spirit of Man, thou mystic plant of grace,
Trefoil of reason, will, and feeling,
How subtle soft the Lyre thy growth revealing
Up to the light of God's own face!
How stern and sweet the Lyre thee, World of Man, revealing!
"The Lyre" by Thomas Aird
And kind Providence once more has sent the sweet, refreshing rains.
The trefoil and the barley grass wave high upon the plains,
The tanks all overflowing and the saltbush fresh and green,
It's a pleasure for to ramble o'er the plains of Riverine.
"The Plains of Riverine" by Anonymous Oceania

In news:

Bird's-foot trefoil (non- bloating ) hard to establish and short lived.
Most intriguing was a black incense burner, depicting a man wearing a distinctive headdress, marked by a trefoil shape on its forehead like the tassel of a jester 's cap.
Trefoil Marine successfully trialed its newest "wide-body" version of its Tri-Air line of high performance craft.
Trefoil Marine successfully trialed its newest " wide-body " version of its Tri-Air line of high performance craft.
The keys are gold, silver, and platinum, with heads shaped like ovals, squares, trefoils, quatrefoils, and hearts.
I'm talking about Tagalongs, Samoas, Do-Si-Dos, Trefoils and, everyone's favorite, Thin Mints.
Less than an hour before a public meeting Wednesday, the Girl Scouts of Southwest Texas notified the city's Planning Commission that it was withdrawing an application to change North Coker Loop to Trefoil Lane.

In science:

Recall, a knot satisfies property P if non-trivial surgery on it yields a manifold with non-trivial fundamental group.) (2) Ozsv´ath and Szab´o’s proof that the unknot, trefoil and figure eight knots are all determined by surgery [37, 38].
Lectures on Contact Geometry in Low-Dimensional Topology
That is, if r Dehn surgery on K is the same as r surgery on the unknot, trefoil or figure eight knots then the knot was the unknot, trefoil or figure eight knot.
Lectures on Contact Geometry in Low-Dimensional Topology
The dehn surgery characterization of the trefoil and the figure eight knot. arXiv:math.
Lectures on Contact Geometry in Low-Dimensional Topology
X,B ) (Unknot) is perhaps the easiest proof that the trefoil is nontrivially knotted.
Bikei, Involutory Biracks and unoriented link invariants
If we want to include the unknot then we can make a reduced diagram of the unknot with at least 3 crossings e.g. simply change one of the crossings in a 3 crossing diagram for the trefoil.
A Second Order Algebraic Knot Concordance Group