It is a soft, illuminating light that awakens Montag's soul: on page three, Clarisse is described as follows:

Her face, turned to him now, was fragile milk crystal with a soft and constant light in it. It was not the hysterical light of electricity but-what? But the strangely comfortable and rare and gently flattering light of the candle. 

The word "cobra" on page six to describe the stomach pump is also connotative. Objectively, a cobra is just a snake and the word, on one level, simply describes the long, narrow quality of the tube put into Mildred's stomach. The word "cobra" also carries the connotation of a dangerous animal that will strike out unexpectedly with sharp fangs to bite and poison a person. Thus, even a hospital, normally a place of healing, is described with ominous overtones, contributing to the novel's dystopic mood.

The word "hound" also has connotative meanings. Denotatively, the hound in the novel is just a machine that the firemen use to help fight enemies, but the word "hound" itself carries the alternate meaning of harass, and its emotional resonances go even deeper than that: hounds historically are used en masse in hunts against a single, helpless creature like a hare. The mechanical hound in this story is not simply a tool of law enforcement, but rather a malevolent creature who adds to the dystopic quality of the novel. On page 11, the Hound is described as

like a great bee come home from some field where the honey is full of poison wildness, of insanity and nightmare, its body crammed with that over-rich nectar and now it was sleeping the evil out of itself.

Bradbury could have as easily created a mechanical killing poodle, but the word "poodle" would not have carried the same connotation as hound.

Connotation is a secondary or associated understanding of a word or phrase.  For example, the word religion can carry a number of diverse connotations for a lot of people.  Some people might feel religion connotes warmth, affection, and family.  Others might think it connotes ignorance and bigotry.  

Denotation on the other hand is simply the actual definition of a word. For example: Dalmatian.  It's a breed of dog that is white with black spots.  For some people the word "dalmation" might have a connotation, like that they are the dogs that are always with firemen.  

Speaking of firemen, Fahrenheit 451 is a book about firemen.  Right from the outset, the reader recognizes that the denotation of firemen is people that put out fires.  The connotation is that firemen are "good guys" that are brave, strong, and selfless.  They help old ladies and rescue cats out of trees.  

Bradbury flips all of that on its head when he tells the reader that firemen are in charge of destroying books by burning them.  Firemen are not brave and selfless wonderful people.  In Fahrenheit 451, the firemen are the antagonists of learning and knowledge.  

Connotation is defined as follows:

Suggestions and associations which surround a word as opposed to its bare, literal meaning. 

Bradbury's Fahrenheit 451 uses connotations masterfully, providing insight that goes beyond what is printed on paper.

In "The Hearth and the Salamander" section of the novel, Montag experiences an assault on his senses that comes from the full-length wall TVs.

 A great thunderstorm of sound gushed from the walls. Music bombarded him at such an immense volume that his bones were almost shaken from their tendons; he felt his jaw vibrate, his eyes wobble in his head. . . .

The thunder faded. The music died. . . .

It was indeed remarkable. Something had happened.

Thunderstorms are literally enormous, earth-shaking events in nature. In this case, the thunder is not only used to describe the activity taking place on the screen, it also describes what is happening within Montag himself. While he mentions that something has happened on the TV when he feels "drowned in music and pure cacophony," the connotation is that something within Montag's world is beginning to shift, also in a gigantic way.

Ironically, the experiences shared on the TVs are meaningless and senseless, but the transformation to Montag's sense of self and his worldview will fuel his personal sense of rebellion. He will eventually be unable to resist in breaking with society's rhetoric and its expectations of its citizens.

Let us remember that connotation refers to the words, ideas and images that come to our mind when we look at a particular word. It is of course a key skill of authors and the kind of words they select and use deliberately to produce and create a particular impression or idea in our minds. One of the biggest examples of connotation in this novel is of course with the words "fire" and "burn." We automatically think of fire as being something destructive, cleansing and dangerous. Yet also the word "burn" has connotations of cleansing and purification.

We can see from the very first lines of this book how these connotations are linked to the theme when the narrator describes the "special pleasure" that Montag takes in seeing "things eaten, to see things blackened and changed." Fire has the ability to transform one thing into something completely different and to "bring down the tatters and charcoal ruins of history." Through this connotation, the strength of fire is something that is established.