gain: the material advantage a person gets, most often profit seized by greed, fraud, or violence
At its core this word simply means what someone gains, the profit or advantage that comes to them. But in the vast majority of its uses the gain is tainted: money extorted, taken by force, or grabbed through dishonest dealing. So English Bibles reach for phrases like unjust gain, dishonest gain, ill-gotten profit, or filthy lucre, and the prophets turn the word into a standing charge against grasping leaders who bleed the people for personal profit.
The same word still keeps a plainer, morally neutral edge in one recurring setting: the blunt question what is the profit? or what do we get out of it? There the speaker is weighing whether a course of action pays off at all, with no built-in verdict that the gain is wicked.
A third shade is concrete rather than moral: the gain taken in war, the plunder or spoil carried off from a defeated enemy. All three run back to one idea, the thing a person comes away with, while the context decides whether that thing is honest profit, greedy loot, or battlefield spoil.