Yet another example of Webster being wrong--or rather incomplete.
Latex, strictly speaking, is more a state of matter than a thing. The word derives from the Latin for milk, and is used to describe a mixture of very small insoluble particles suspended in another liquid.
Milk, for example is a latex. It is a suspension of fat in water.
Latex paints are another example. They are similar to emulsion paints: a suspension of insoluble stuff (pigment) in water. They do not contain any natural rubber latex.
It is a common mistake to confuse "latex" with "natural rubber latex". The latter term has become almost synonymous with the former, but it is still important to make the distinction between the two.
Natural rubber latex (NRL for short) is indeed a latex, but it is not the only one.
The plural form of latex is latices (say it, "lay-tiss-eez")