It's an electrolytic capacitor. These are polarized, as the
+ sign also indicates. This is a less common symbol. Below are the more common ones, European on the left, American on the right.
Compare to the symbol for a non-polarized capacitor:
Note: I think the American symbol for a non-polarized cap is a bad one; it suggests that there is some kind of asymmetry where in reality there isn't one.
From the comments it appears that the supposedly American non-polarized symbol is less common than I thought. I can only speak from my experience, and like I also said in comment, it could be that I've been looking mostly at older schematics (not the tubes, I'm not that old).
I found this schematic within a minute:
C2 might be an electrolytic (it won't be, will have a too low capacitance), but look at variable capacitor C1.
Also this page.
Browsing through more symbols encountered also this weirdo (the one on the right):