Money or profits. -- American Heritage Dictionary
American Heritage also offers this pretty interesting word history:
Word History: When William Tyndale translated aiskhron kerdos, "shameful gain" (Titus 1:11), as filthy lucre in his edition of the Bible, he was tarring the word lucre for the rest of its existence. But we cannot lay the pejorative sense of lucre completely at Tyndale's door. He was merely a link, albeit a strong one, in a process that had begun long before with respect to the ancestor of our word, the Latin word lucrum, "material gain, profit." This process was probably controlled by the inevitable conjunction of profit, especially monetary profit, with evils such as greed. In Latin lucrum also meant "avarice," and in Middle English lucre, besides meaning "monetary gain, profit," meant "illicit gain." Furthermore, many of the contexts in which the neutral sense of the word appeared were not wholly neutral, as in "It is a wofull thyng . . . ffor lucre of goode . . . A man to fals his othe [it is a sad thing for a man to betray his oath for monetary gain]." Tyndale thus merely helped the process along when he gave us the phrase filthy lucre.