In Victorian novels, potential husbands or wives are judged by how much they ‘get a year’ - for women, who were obviously not employed, where did this money come from?

Capt C. Why, you won't face me down. - Did you not just now say she had ten thousand pounds in money, five in jewels, and a thousand a year?

(From Act I, scene i, The Tender Husband, 1824)

"Well, I will be serious," said Laurence in a slightly less bantering tone. "Be just; or, if that is too high a flight for your ethical wings, be good natured. This marriage is for your good as well as mine, yet I am to be the only victim. Grant me at least the luxury of kicking while you harness me. Now let us go fairly through the available list. Miss Sefton?" He laughed, but it was not quite a natural laugh, and, strangely enough, he whose general look was fixed and steady now kept his eyes bent down, intent on the condition of his nails. "She has money, I believe," he added in a jeering kind of way. "Fifty pounds a year, if a penny."

(From "Black Tarn", serialized in All the Year Round, 1860)

But it was not of Sylvia that he was thinking, as I discovered, for he began to ask me questions about Mrs Glen-Warrington.

"I'm told she has twenty thousand a year. I know nothing," I said; "but every one gossips about his neighbour or his stranger. I never set eyes on the lady before."

(From "Lady Sylvia", in Pall Mall Magazine, 1901)

Women who had so-many pounds a year in the nineteenth century had family money. It could be that they were heiresses who already had money under their own control, in which case the capital is most likely invested in the government funds, with either a four or five percent interest rate - the interest is the income the heiress is said to have per year. They might also be said to have so much per year based on an expected inheritance or their expected dowry, if it were invested in the same way, if they were not yet independent. (While potential husbands being judged this way could have had their income from their occupation, anything over £500 most likely also refers to income on invested inherited wealth in this context.) You can read more details about this in Jane Austen in Context (Cambridge University Press, 2005), but it's fairly straightforward.

I'm sorry this isn't very long - your question is really about a definition, and we should have probably referred you to the Short Answers for Simple Questions thread.