You can speak of ‘premature’ if you come before you or your partner wish it to occur. Or you can call it premature if it takes less than a certain time, for instance a minute (with the average man it takes 2 or 3 minutes). Some call it premature if ejaculation happens before the woman comes to orgasm. But the real issue is whether or not you have voluntary control. Some guys are able to determine the moment of orgasm, while others are not.
The cause of premature ejaculation is still a mystery. But research has at least made it clear that premature ejaculators often behave differently than guys who can easily control the moment of orgasm: they don’t make the adjustments in behaviour necessary to postpone orgasm. Possibly because they are too focused on their lover to think about it or because they simple don’t know how to do it.
If you always come rapidly you might want to learn how to improve control.