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Flash Loan: What Is It? Definition & Explanation

A flash loan is a DeFi loan that requires no collateral and must be borrowed and repaid within a single blockchain transaction. If the loan is not repaid in the same transaction, the entire transaction is reverted as if it never happened.

A flash loan is one of DeFi''s most innovative and also most abused tools. It makes possible something impossible in traditional finance: borrowing millions of dollars with no collateral and repaying it within the same transaction, all in seconds.

What Is It?

A flash loan is a loan type built entirely on smart contracts. The core rule is this: the loan must be repaid by the end of the blockchain transaction in which it was taken. If repayment does not happen, the blockchain''s atomic nature reverts the entire transaction — meaning the loan is treated as if it was never issued and no step ever occurred.

How Does It Work?

A user takes a flash loan from a protocol such as Aave or dYdX. Within the same transaction, they use those funds for a series of operations (for example arbitrage, collateral swapping, or liquidation). In the transaction''s final step, the borrowed amount plus a small fee is repaid to the protocol. All of these steps happen in a single atomic transaction; if any one fails, all of them are reverted.

Why Does It Matter?

Flash loans let even users with no capital perform large-scale arbitrage, collateral optimization, and debt refinancing. This is a unique feature that distinguishes DeFi from traditional finance and improves market efficiency.

Risks

Flash loans are the most common tool in DeFi exploits. Attackers use massive uncollateralized funds to manipulate price oracles, drain liquidity pools, or carry out governance attacks. So while the flash loan itself is a neutral tool, it poses a serious threat to poorly designed protocols.

FeatureDetail
CollateralNot required
DurationSingle transaction (atomic)
RepaymentMandatory in the same transaction
Common UseArbitrage, liquidation
Primary RiskOracle-manipulation attacks

Diagram showing the borrow, use, and repay steps within a single atomic transaction

COINOTAG Perspective

For COINOTAG, flash loans represent both the power and the fragility of DeFi. A protocol''s resilience to flash loan attacks (for example robust oracle design) is a critical indicator for assessing that protocol''s security maturity.

Last updated: 6/21/2026

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