A vinyl compound is any organic compound that contains a vinyl group (also called ethenyl), −CH=CH2. These are derivatives of ethene, CH2=CH2, with one hydrogen atom substituted with some other group.
A related phrase is vinylidene which stands for a double-bonded two-carbon residue with two rather than one substituents, for instance in 1,1-dichloroethene ("vinylidene chloride").
The vinyl group can be made to polymerize, forming vinyl polymers. In these polymers, the double bonds of the vinyl monomers turn into single bonds and the different monomers are joined by single bonds. This is an instance of addition polymerization. There are no vinyl groups in the resulting polymer. It is also important to ascertain the absence of unreacted vinyl monomer in the final product when the monomer is toxic or reduces the performance of the plastic.