What is the meaning of the title "A Sound of Thunder" by Ray Bradbury?

The title is a phrase taken from the narrative of Bradbury's short story; it is used twice. The first time it is a description of the footsteps of the Tyrannosaurus Rex that Eckels has contracted to shoot, and the second time it is a description of the sound of a deadly blast from a rifle. Thus, it has two meanings.

• First Meaning

After the Time machine takes Eckels back a million years, he finds himself in a jungle which is full of various sounds, such as "twitterings, rustlings, murmurs, and sighs"; however, as the great dinosaur approaches, all the creature that live in this jungle stop what they are doing. 

Silence. A sound of thunder. Out of the mist, one hundred yards away, came Tyrannosaurus Rex. "It," whispered Eckels. "It......  

As the monstrous creature marches through the jungle, the activities of all the other occupants are arrested in trepidation. The giant has the jungle to itself while it walks with such weight and strength that it makes sounds as powerful as the reverberations of thunder.
It is this terrifying sound which causes Eckels to recoil in fear.

"It can't be killed," Eckels pronounced this verdict quietly, as if there could be no argument.

When Eckels sees this prehistoric giant, he is so intimidated by its power that he feels that the rifle he holds is like a mere cap gun. He tells his guides, "We were fools to come. This is impossible." These remarks infuriate his guides, who have to shoot the giant reptile themselves. They order the frightened Eckels back into the Time Machine and tell him to walk quietly and carefully. 

• Second Meaning

Earth-shaking changes occur as a result of Eckels' trip because he has stepped off the gravity path which prevents anyone from making any changes to the environment into which they time travel. Unfortunately, Eckels has stepped upon a butterfly, killing it.

When Eckels and the others return to the present time and the office of Time Safari, Inc., there is a different person behind the desk, words on the sign are indecipherable, and there is a chemical taint to the air. Eckel senses other changes, as well. So, fearfully, he examines the bottom of his shoe:

Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful and very dead.

Eckels falls to his knees. Trembling, he asks if there is not some way that the butterfly can be brought back to life, and reverse things to the way they were before he left on the adventure. "Can't we start over?" he asks in desperation. Frozen in fear, Eckels waits.

....He heard Travis [one of the guides] breathe loud in the room; he heard Travis shift his rifle, click the safety catch, and raise the weapon. There was a sound of thunder.