As a verb, it means to put money into a bank account or to put something down in a particular place. As a noun, it can mean money placed in a bank, money given as a guarantee, or a natural layer of material left by a process, like a mineral deposit.
From Latin *deponere* meaning 'to put down, to lay aside', from *de-* ('down') and *ponere* ('to put, to place'). The idea of putting something in a safe or fixed place led to both financial and geological meanings.
Whether it’s cash in a bank or sand at the bottom of a river, a deposit is something 'put down' to stay for a while. That’s why a 'security deposit' is money you put down to anchor trust between you and a landlord.
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