( I meant to type BlaYlock, is there gonna be a family fued?) Glenn, When thunder is used to describe another sound it is being used metaphorically or compared to actual thunder. Not really an alternate definition. "Thunder" specifically means "sound caused by lightning." The closest thing to real thunder is a "sonic boom" and thunder is closely related, a sonic boom however is caused by the actual parting of the air by a physical object, the plane, and then 'clapping' back together behind it. It used to be thought that thunder was caused the same way. As Glenn said, it is the actual energy of the lightning that causes a rapid expansion or"'explosion." There is bit of physical in thunder and a bit of the energy cause in a sonic boom.The etymology is kinda strange, there is an ancient Persian word "tundar" which means thunder, it can be traced way back to even more ancient Indo- European roots, the English word seems to come directly from Scandinavian and Germanic words. No one seems to be able to connect the two roots. In Swedish "tordon" translates directly as "Thor's din" and Thor is the god of thunder. It may be that instead of thunder being named after Thor, Thor was named after thunder. In German the word is donner as in Donner and Blitzen or "Thunder and Lightning." Dreamer, I grew up on the Great Plains and have seen and enjoyed many lightning storms. ( Mmm a few times maybe it was a bit more apprehension than enjoyment.) Not only have I experienced lightning "without' thunder but the other way around too. There is also what is called "Heat Ligtning" , which is really just regular lightning happening so far away you can't see the actual bolts, sometimes not even the clouds, just a flickering on the horizon, and so far away you don't hear the thunder. When you don't hear thunder from nearby lightning it is because of atmospheric conditions, there is so much moisture in the air that it dampens the sound and the lightning is not as nearby as it seems but miles high. That can look spectacular at night with the clouds and apparently the entire sky just lighting up like a giant strobe light. Sometimes there is thunder without any lightning, usually during the day. There is a low layer of thick clouds that blocks the light from lightning happening way above it, but you hear a constant low rumble and/or low booms, and occasional crack. My favorite thunder happens when a miles long bolt passes low over head from horizon to horizon, after it's gone you hear a low rumble getting louder, and closer you can hear it moving not just getting louder, then a crackling and snapping, then loud thunder and then an incredible explosion KAWHAM! that shakes windows and walls and even the ground and then it rumbles off into the distance, all in a split second.

The sound of "Thunder" (not just sounds like thunder) can only come from a lightning strike. However, the sound that is given the name 'thunder' is basically the same as a sonic boom. In fact, that is all it is. The incredible power of a lightning strike superheats the air around it, causing a rapid expansion that pulses out in a shockwave, at the speed of sound. This shockwave is composed of pressurised air particles and a great amount of sound waves. When this wall of sound passes over us, we hear the loud boom of thunder. Because light travels at the speed of light, 300,000km/sec, and sound travels at the speed of sound, approx 1,220km/hr at sea level, the light will reach you before the sound does. That's why there is a delay between seeing the lightning flash and then hearing its accompanying thunder. ____ Also, sonic booms in aircraft are not caused by the air separating and then clapping back together. When an aircraft approaches the sound barrier, it causes a pressurised wall of air and sound waves in front of it. Since the sound waves are travelling at Mach 1 (the speed of sound), a source travelling at the same speed will cause the sound waves to 'pile up' in a shockwave. The effect of this shockwave passing us is what we know as a sonic boom. When the aircraft punches through this shockwave (i.e. goes supersonic), it causes an effect known as a sound cone. Imagine a large, horizontal cone with the plane at the very tip. The waves of sound move slower than the plane, so it effectively leaves them behind as they spread out further. This forms a cone of sound waves, because the ones at the back have spread out further than the ones closer to the source. Hence, if you're standing on the ground and a supersonic aircraft flies over you at 1,000 feet, you will see the aircraft before you will hear the boom. The higher it is, and the faster it's going, the longer the distance between the plane passing and the shockwave passing.

Thunder is the sound of lightning happening. So without lightning thunder wouldn't exist

I'm not sure about the first part of your question, but to answer the second half, yes one can occur without the other

i must say simply that the above answer is half bull the rapid expansion of air does in fact NOT make the sound of "thunder" but rather the collision of these air molecules colliding together after their rapid expansion that is what we call "thunder"