Definition
A kimono sleeve is a sleeve cut in one piece with the bodice, with no armhole seam, giving a loose T-shaped garment with a soft, sloping shoulder.
In Western patterns, kimono sleeve means a sleeve that grows out of the bodice: the shoulder seam continues down the top of the arm, and the side seam turns into the underarm seam in one curve. There is no armhole and no sleeve to set in, which makes kimono-sleeved tops and dresses among the fastest garments to sew.
The trade-off is movement. The lower the sleeve angle, the closer to the body, the more fabric bunches under the arm; the higher the angle, the more the garment lifts when you raise your arms. Loose, boxy styles accept this, and fitted versions add a gusset at the underarm. Reinforce the curved underarm seam with a short stitch length or stay tape, and clip it so it lies flat.
The traditional Japanese kimono is built differently: its sleeves are rectangles sewn to a straight-cut body, with a deep opening under the arm. Modern kimono jackets follow the same idea. The kimono jacket has wide rectangular sleeves sewn to a T-shaped body, which keeps cutting and sewing simple.
The dolman sleeve is a close relative with a very deep armhole that may reach the waist. Compare with the drop shoulder, which moves the armhole seam down the arm, and with the raglan sleeve.