The term BBQ (aka barbecue, barbeque, bar-b-que, etc) has been so mangled by amateurs as to become nearly meaningless. BBQ is commonly used as both verb and noun, and applied to everything from hot dogs with sweet ketchup to smoked hams to the actual grill. The truth, as you already suspect, is somewhat more restrictive.
Texans will disagree (I could stop the sentence here and this would be one of life's great truths), but BBQ is pork or chicken slow cooked in a covered grill over a wood fire for between 8 and 18 hours and seasoned with some combination of salt, pepper, and other spices that vary by region. Cooking times are obviously shorter for chicken, with 2 to 6 hours being the defining range.
BBQ is NOT hot dogs or sausages cooked in 20 minutes on the hibachi, ribs cooked in the oven with a ketchup based sauce, or steaks on the grill. These are all fine foods, but none qualify as BBQ.
Beef, fish, and vegetables (an exception is made for goat and wild game) cannot be BBQ. They can be grilled, roasted, smoked, baked, broiled, steamed, and prepared a thousand different ways, but they simply cannot be BBQ. I am a tremendous fan of smoked tenderloin and I love brisket, but BBQ doesn't come from a cow.
So who am I to pronounce such a definition? Thats a fair question, and one I'll answer briefly. I grew up on a farm in the Bootheel of Missouri, where BBQ is revered as a cultural icon in the same way as creole cooking is revered in New Orleans. By the time I had my 20th birthday, I had established a substantial reputation as a BBQ chef, and built a small business catering BBQ and providing BBQ to local restaurants that didn't have their own grill. After college I began to enter organized BBQ competitions (Chief cook for Bubba Q. and the Beans) and built a collection of delightfully tacky trophies and plaques from BBQ contests in TN, MO, and AR.
After a few years, I opted to judge BBQ contests rather than compete (less work, better treatment), and had the honor of judging in a number of regional BBQ contests and the MIM International BBQ Cook-off - the World Championship. During this same time I hosted a weekly food show on WEVL Memphis and annually picked the top 5 BBQ restaurants in the Mid-South. Judging and reviewing exposed me to some of the finest BBQ chefs in the country, and I gained a deeper understanding of the art.