You are using Machine Learning techniques to build a model that will predict what email messages coming into your inbox are spam messages. The model you've created to make this determination is based on the sender$\text{'}$s email address, the appearance of certain words in the subject line $($e$.$g$.,$ "winner", "click", "deal", etc.$),$ the total size of the message in KB $($kilobytes$),$ and whether or not the email has an attachment. Again, given that we are working in the context of Machine Learning, what would any one of the factors in the proceeding list be called?