According to the Oxford English Dictionary's definition, theme is "the subject of a piece of writing."
Now, that might be factually correct (who am I to argue with the dictionary people?), but I still don't believe it is helpful to anyone just starting out in novel writing who is trying to work out what theme actually is.
The "subject of a piece of writing" suggests "subject matter" - and, for me, a novel's subject matter is something concrete and definite.
So the subject matter of a horror novel, for example, might be vampires and spooky castles.
A literary theme, on the other hand, is not concrete at all. It can usually be summed up by a phrase like "grief" or "unrequited love" or "blind faith" - something intangible like that.
So let's start again...
What is theme? According to my own definition, the theme of a novel is simply what a novel "means."
I think I've said somewhere else on this site that a novel's theme is what it is "about." Thinking about it, though, that sounds dangerously like "subject matter" again, which is why I am defining theme here as what the events of a novel "mean."
A love story, for example, is "about" two people meeting and falling in love. In other words, it is about...
...and these things are all on the novel's surface. In fact, they are the novel's subject matter.
The theme of a novel, however - or the meaning of a novel - happens beneath the surface, and it is essentially the lesson that the surface story teaches us, or the conclusion that can be drawn from the events.
If all of this is sounding kind of vague, that's because theme in fiction is vague. But we are slowly edging closer to a more concrete definition...
The theme of a novel is the deeper layer of meaning running beneath the story's surface.
If you are still confused, I'll try to un-confuse you by looking at that love story I mentioned in a little more detail...
Now, everything I talked about was on the story's surface - the characters, plot, and setting, in other words. I didn't mention theme once - but it is there, nevertheless.
How you choose to interpret the meaning of a story is sometimes a matter of opinion (different readers can take different things away from fiction).
But, for me, the lesson to take away from the John and Sarah tale (or the conclusion to be drawn from it) is simply this: Love without trust can never work.
So far, we have defined theme as being what the events of a novel "mean" (or what the novel is "about" on a philosophical level).
Now, this "understanding of the human condition" does not need to be psychological in nature. It can be political, sociological, whatever you like.
Here is a selection of issues that you might write about in your own fiction...
"Theme is a statement, or series of related observations, about some aspect of the human condition, interpreted from the unique viewpoint of the author."
- Dean Koontz
"The theme of art is the theme of life itself."
- Lawrence Durrell
Something worth noting is that it isn't necessarily your job as a writer to provide answers, but merely to ask questions.
If you are strongly opposed to the death penalty, for example, so much so that you want to base a novel around that theme, you could write a story about an innocent man (or even a guilty man) on death row.
The way you construct your story will subtly but forcefully make your position clear: that death as punishment is wrong!
But if you feel strongly about the issue but aren't sure which side of the debate to come down on, you could write a novel setting out both sides of the argument and leave it to the reader to reach their own conclusions.
Your novel will ask questions, but it won't provide answers.
You could do something similar with a more psychological theme in a novel, like guilt. Is it better to live with the weight of guilt on your shoulders, or to come clean and face the consequences?
You really don't know.
So you could explore both possibilities in your novel, perhaps through different viewpoint characters, and let the reader take from the novel what they will.
If you've made it this far down the page, congratulations. I'm conscious of having rambled a bit, but everything above is about as concrete an explanation of theme in fiction that you are likely to find.
Here is my final definition...
The theme of a novel is the deeper layer of meaning running beneath the story's surface. While the surface story entertains the readers, the theme helps them to reach a new understanding of some aspect of the human condition.
Or to put it another way...