Liners or Septa?
A liner can be made of the same material and be the same or different thickness as a septum but it is installed in a solid top cap and therefore cannot be pierced as Septa do. Its primary function is to keep the sample from escaping the vial while the cap is on.
Note: According to Wikipedia a Septum (Septa is Plural) is Latin for something that encloses; or a wall, dividing a cavity or structure into smaller ones. For Chromatography, a Septum encloses the vial keeping samples and gases enclosed in the vial until a needle penetrates it and extracts some of the sample to be injected and then reseals itself.