In my opinion, it’s all relative, really.
Here is my definition of falcetto: The condition of the vocal folds whereby the glottis is large and a lot of air is passing through.
Here is my definition of head voice: The condition of the vocal folds whereby the glottis is small(er), and the folds are able to withstand more breath pressure.
Let me explain.
With my own voice, I basically consider my falcetto to be a light head voice. This is the condition where I am allowing more air to pass through on a high pitch.
With some singers, this condition happens when they reach a certain pitch whereby the vocal folds cannot withstand the amount of breath being released. Basically, they blow apart.
In other singers, this condition happens as they gradually get higher and higher in pitch. There may not be an actual sensation of flip, but rather a breathiness that comes with singing high notes.
Ideally we want to have good vocal cord closure on our high notes without over-compression (squeezing). Singing in your falcetto (or your head voice, whatever you decide to call it), is an important element to becoming a better singer.