dude, that's a German Web site. Here's an American one:http://www.merriam-webster.com/dictionary/jar...
M-W seems to have to have got its definition from the OED (as I suspect it frequently does):
"JAR: A vessel of earthenware, stoneware, or glass, without spout or handle (or having two handles), usually more or less cylindrical in form. Orig. used only in its eastern sense of a large earthen vessel for holding water, oil, wine, etc."
Now, I'm not going to bother with a google search, because there is, no doubt, no end of people who say "plastic jar" (and no doubt "plastic box," "plastic tub," "plastic can," "plastic vessel," etc. as well). I wish only to confirm the traditional and most usual meaning of the word jar.