According to https://en.wikipedia.org/wiki/Variance variance is related to random variable. Random Variable is defined in terms of sample space and probability measure function.
In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its mean.
The https://www.mathsisfun.com/data/standard-deviation.html site defines variance without any relation to random variable.
Does this mean variance is a property on the set of data and since random variable represents a set of data, variance is defined over it ?