Third eye lid

Hi all,
What exactly is the third eyelid? My guess is that it's underneath the eyelid and is exposed? My little girl has this some days - pretty randomly. It's white, and kind of sticks out below her eyelid.... does this mean she's not well?? She's acting absolutely normal. I just ignored it until I read in some posts about the third eyelid. Any ideas?
Thanks!

*Usually* the appearance of the third eyelid signifies that the cat is starting to have a health issue. Sometimes it is just something mild like an eye infection but other times it can come right before a fever or other serious illness. It is also called the nictating membrane and is visible also when the cat first wakes up or when it is deleriously happy (like during a catnip fest). It helps to keep the eye lubricated and wards off dust-

Thanks Hissy!
So I realize that what I'm talking about possibly isn't the third eyelid. Hers comes down underneath the eyelid up top.... not the same thing, right?

http://maxshouse.com/Ophthal_Pics/El...s_syndrome.gif

http://www.cvm.missouri.edu/neurolog...uton/floyd.jpg