Electronic – Is this a varistor

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.

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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.

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Browsing through more symbols encountered also this weirdo (the one on the right):

One of the main differences between a TVS and a Varistor is that the voltage tolerance of a TVS is much tighter than a Varistor. They also don't appear to have the "wear" factor that Varistors have.

Varistors that are repeatedly subjected to transients appear to have their clamping voltage lowered as a result of the stress. Eventually, they become conductive at the operating voltage and then fail catastrophically. That is: they get very hot, then turn black, then eventually catch fire.

Many of the better AC power bars that have Varistor clamps within also have a thermal cutoff device mounted between two Varistors. The thermal cutoff device disables the power bar when a Varistor fails.

In terms of what happens to a transient suppression device when it is exposed to voltages in excess of their rated clamp voltage: they get hot. The will, in fact, get hot enough to fail.

When a Varistor fails because of over-temperature, it generally becomes restively-conductive. That is, it turns black, then catches fire. Your over-current device may or may not operate, depending on how low the resistance of the Varistor has dropped.

When a Tranzorb fails because of over-temperature, it generally becomes a short-circuit. This causes sufficient current to flow to cause your over-current device to open.