In over 4 decades of hand painting coats of arms for folk, I have come across the term "family crest" many times, and indeed have had to resort to using the same words myself to be understood. Most people know that the correct way is actually coat of arms, but may not realise the importance of the actual crest in a coat of arms. Nowadays the many big companies selling printed heraldry products online simply ignore the crest part of the arms altogether, or put a bunch of 2 tone feathers (usually 5) over the helmet. Not  only is this lazy, it is incorrect, as the majority of coats of arms have a proper crest, usually much more interesting than a bunch of feathers! The reason the helmet is depicted on a coat of arms is because that is where the actual crest was placed. The crest consisted of actual stuffed animals, arms or hands,(made from leather) holding daggers, or other organic materials such as tree sprigs. The purpose of the crest was to make the knight appear about 7 feet tall in the saddle. There can be little doubt that the sight of an armed warrior on horseback with a boar's head on his helmet coming at you would be cause to worry....