A sleeveless, is a diminutive for a garment (obviously a soutien-gorge) that is devoid of sleeves. As with the shirt is topological controlled by four edges--the neckline, cutoff line and two armholes. It may or may not have front or back closures, and it can be a vest (ET6401). And it may or may not expose bellage--although if it does it would more typically be called a croptop (RW196310). So in this respect, certainly in the thinking of Roland Barthes, the sleeveless is more an adjective or costume, a modifier, than a pure species.
Origins, Armpits, & Armholes
The sleeveless emerges in fashion wear once the arms become bare in the early 1900s and remain so thereafter. It is a garment in which the sleeve length is zero, but also, by definition, not strapped.
It is important to note that the sleeveless bares not only the entire arm, but permits the exposure of the armpit as well, an important erogenous zone and one which is dependent upon the movement and positions of the voyeuree (VB8421). The shape of the armpit varies with the body weight of the subject, muscle tone, and movement, and can range from rounded to deeply concave.
The sleeveless may be engineered with a variety of armhole lines, from the tightly fitted (NYA20514), to the normal (WG7312-13) to the excessively wide. This mid-1940s croptop hints at a sleeve (LT4620), this one sits on the shoulder (EPA090). At one extreme the armhole can migrate upward to the neck and backward across the shoulder blade into the silhouette of the T-back. On the other hands it can increase in radius in the front and below the underarm so as to provide unencumbered views of any bra or slip underneath, or the bare breast and nipple if so desired (SM8528). This kind of cleavage is called cleavage côté.
The exposure of underwear within the armhole, including the bra strap, cup, and even the side support and skin below it, has been considered vulgar in many periods, but as the 1990s progressed the casual display of underwear became trendy, if not vogue. Thus the armhole, like the neckline, is a primary vector toward breastage and nippage.
The coyness of the sleeveless exposures also plays a role in the venue of the bikini. Because the sleeveless is normal streetwear, it provides "coverage" when a bikiniite wants to push the limits of venue, as with this tanga and v-kini couple at a very non-beach location (LV8401-02).
If one vector away from the sleeveless is toward a shortsleeved and longsleevd soutien-gorge, then opposite direction lies in the strapped soutiens-gorge including the undershirt, camisole, brassiere, and T-back. Like the sleeveless blouse, the undershirt and camisole are certainly sleeveless--but once the fabric between the neck and shoulder shrinks to a strap the nature of the garment is sufficiently changed so as to classify as a different species. A similar argument can be made for the T-back, as well as the halter, which in fact doesn't have armholes at all.
Any sleeveless can also be a croptop, and visa-versa, so in this respect both are modifiers of the other. More precisely, the each modifies a different fashion line of the garment, with the sleeveless a mid-point on an "armhole" that migrates from longsleeved to shortsleeved to sleeveless to strapped, haltered, and strapless (the latter two not having armholes at all).