Function is meaningful movement, it is not an isolated event, and it is movement that is leading toward something not an end unto it self. Movements that are less functional are movements that repeat themselves and are isolated. For example the leg extension or leg curl repeat themselves, they are essentially an end in themselves. Contrast this to the lunge that is progressive and can lead to many variations. The body is a link system and movement involves the timing of the movement of the links of the kinetic chain. It is helpful to visualize the process as total chain training moving from toe nails to fingernails. The outcome is functional sports training which incorporates a global systems approach to training & rehabilitation.
In many ways function defies a strict definition because it always must be defined in the context of the end result. We certainly can see it and feel it. Common sense will direct us to activities that are related to what we are ultimately training for. Since it is about movement it should be instinctual. Unfortunately in the search for quantification and technological advancement we have, at times, outsmarted ourselves. I look at relatively simple criteria; if I am getting too far away from the body and fundamental movement then in all probability what I am doing is not highly functional, it won’t transfer.