Naga
Often confused with the Lamia, the Naga is a creature from Asiatic mythology (specifically, Hinduism) that combines human and snek in its appearance. Nagas are divine serpents, portrayed as capable of shapeshifting between purely human, purely serpentine, and half-way forms; many artwork depicts them as snakes with human heads, though they can and do often have arms - divine ones in particular tend to have more than one pair of arms, given Hindu deities are usually multi-armed.
This origin is why "naga" is often used as an alternative name for the same "snek-centaur" as the typical lamia. When the two races are visually distinct from each other, it's usually achieved by giving the naga "more serpentine features" than the lamia has. Exactly what that means depends on the interpretation.
In Dungeons & Dragons and Pathfinder, for example, Nagas resemble giant snakes with human heads. In the earliest editions, the human heads were very distinctive from the snake bodies, but as editions have gone past, they've become more and more reptilian-faced, and honestly kind of creepy-looking.
In more monstergirl/furry communities, nagas are basically lamias (humanoid upper torso, giant snake's body from the wast down), but with scaly skin on their upper torso as well (normal lamias, like your typical merfolk, are depicted with human skin giving way to scales). Sometimes, they may go so far as to have anthropomorphic snake's heads atop their human-like shoulders, but this is most common amongst furry artists.
Finally, a naga may be described as having an "armless" humanoid upper torso (with said torso potentially overlapping with the aforementioned "scaly-skinned humanoid and/or snake-morph upper torso" depiction). What this basically amounts to is the "human-headed snake" naga, but with boobs, so it's pretty much only seen in monstergirl porn - it allows one to mix "monstergirl" with "amputee" fetishism.