What do we denote when we say that a capacitor has an amount of charge Q on it?

Answer:

If we obtain the expression the charge on the capacitor too accurately the value is always zero because the charge on one plate of a capacitor is always equal to the negative of the charge on the other plate. The expression is physics jargon as well as we just have to remember that it means the absolute value of the charge on either plate. Consequently if one plate has 15 mC of charge on it, the other plate has -15 mC of charge on it and the capacitor is said to have a charge of 15 mC on it.