I am studying the feature selection in the machine learning field. Lately, I am trying to understand the PCA (Principle Component Analysis). I found a webpage PCA which introduces the PCA and the concept of percentage of variance. However, I am very confused about what "Percentage of Variance" means. I am sorry for my shallow statistics knowledge first. Here are the questions I have
(1) Is there a formal definition or mathematics formula to define "percentage of variance" ?
(2) Do we only use POV for calculating PCA ? Does somewhere else use POV ?
(3) Is POV the same as "Proportion of Variance Explained" ? There are many similar terms online which really confuse me.
Thank you very much,