First, there is the voltage rating. The voltage rating on a capacitor is of course a maximum DC (i.e. a peak) rating. For 50/60Hz mains we're talking about a sinusoidal voltage waveform with an RMS value of for instance 230V, so the DC peak value of such a supply is Vrms ⋅ 2–√ r m s ⋅ 2 or about 1.4 times this quoted value.
If there is no AC rating then there is no rating, meaning that you're on your own if you use this capacitor with AC. From the AC voltage (peak, RMS etc.) it can be calculated what the maximum voltage is and that should be lower than the DC rating. However, a capacitor without an AC rating isn't guaranteed to work safely with AC.
So if a capacitor is going to be exposed to 25 volts, to be on the safe side, it's best to use a 50 volt-rated capacitor. Also, note that the voltage rating of a capacitor is also referred to at times as the working voltage or maximum working voltage (of the capacitor).
A capacitor with a 12V rating or higher would be used in this case. In another, 50 volts may be needed. A capacitor with a 50V rating or higher would be used. This is why capacitors come in different voltage ratings, so that they can supply circuits with different voltages, fitting the power (voltage) needs of the circuit.
One very important rating of capacitors is "working voltage". This is the maximum voltage at which the capacitor operates without leaking excessively or arcing through. This working voltage is expressed in terms of DC but the AC equivalent is about only one half of that DC rating.
The AC voltage rating is the maximum Voltage the capacitor can handle divided by the square root of 2. So this capacitor can handle 630V / 1,414 = 445,5V I wouldn't go higher than 400VAC though. Nick - that's for a sine wave. He notes in a comment "I will be using this in the resonant converter which is secondary side ".