Ballerina neckline
A low neckline associated with strapless or spaghetti-strapped garments
Boat Neck / Bateau Neck
A high, wide, often straight, neckline running from shoulder to shoulder. Usually has the same front and back and tapers to a point on the shoulders.
*Cowl Neck
A piece of material attached to neckline that drapes loosely from shoulder to shoulder at front and/or back.
*Crew neck
A round neck that fits close to the base of the neck. In knitwear, crew neck tops may feature a ribbed band around the neckline (e.g., the classic T-shirt).
*Diamond neck
A diamond-shaped cutout that fastens at the front or back neckline. Other cutout neckline shapes include the tear-drop and keyhole.
*Décolletage/Décolleté
A plunging neckline that reveals your cleavage, it’s often used in off-the-shoulder designs
Draped neck
Neck with soft pleats that can be arranged in various ways.
Draped neckline
Neckline made up of a series of soft overlapping folds created by the fabric of the bodice.
*Dropped shoulders
Characterized by the shoulder-sleeve seam falling off the shoulder.
*Florentine neck
A wide square neckline extending to the shoulders.
*Horseshoe neck
A deep, rounded scoop neck that resembles a horseshoe.
*Jewel neck
A high round neckline that rests at the base of the neck.
*Notch neck
A round neckline with a small triangular cutout in the front center.
*Off-the-Shoulder
A neckline which lies just over the top of the bust with shoulders uncovered.
*Portrait
A neckline that frames your shoulders and face while accentuating the bustline. It’s often gathered at the center with a brooch or other ornamentation.
Plunging neckline
Neckline that usually ends in a V and is very low in front or back.
Round neck
Rounded neck fitting close to the wearer’s neck.
Sabrina
A neckline that crosses and covers your collarbone.
Scoop Neck  
An open, low, U-shaped or round neckline.
Square neck
Neckline with a square neck that reveals the upper part of the chest.
Sweetheart Neckline  
An open, curved neckline with a scalloped edge similar to the top of a heart.  
V-shaped neck/back
Neckline with edges coming together at mid-chest in a V shape.
Here are the terms for various collars, neck lines and neck treatments.
Bertha collar
Collar made of a strip of fabric of variable width and attached to the edge of a
neckline or round neck.
Bow collar
Collar made of a long strip of soft fabric sewn onto a round neck; it can be tied in
front in various ways.
Collaret
Piece of delicate, pleated or gathered fabric that adorns the neck of a dress; its
shape has varied greatly from one period to another.
Cowl neck
Turtleneck that is large enough to be draped over the head, making a kind of
hood that frames the face.
Dog ear collar
Turned-down collar characterized by long, fairly wide points, which are rounded
at the tips.
Resources for definitions:   visual.merriam-webster.com    dressking.com   mollybloom.com   edressme.com
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Jabot
Decoration made up of one or two pieces of fine soft pleated fabric; it is attached at the base of the neck and spreads out over the chest.
Mandarin collar
Stand-up collar with rounded upper points that come together at the neck, forming a V.
Notched collar
A two-piece collar that can be only worn open.
Peter Pan collar
Flat collar of uniform width with rounded tips; it is sewn onto a fairly open neck.
Polo collar
Turned-down pointed collar fastened with a buttoned placket, which ends at mid-chest.
Sailor collar
Has square flap in back and long lapels extending over the chest, fastened to a V-neck. Out of modesty, the plunging neckline is often concealed with a piece of fabric.
Shawl collar
Wide turned-down collar with long rounded lapels that partially cross in front.
Shirt collar
Collar with rounded or tapered points that is sewn onto the neck and turned down along a fold line, which is higher in back than in front.
Stand-up collar
Collar made of a narrow strip of fabric that sticks up from a round neck; its edges meet in front but do not fasten.
Tailored collar
Collar whose fold covers the back of the neck; its lapels form a V where they cross on the chest.
Turtleneck
High-necked collar that is folded over; it is usually snug around the neck and does not fasten.
Wing collar
A collar that covers the shoulder seams of bodices.