Collusion is a form of market manipulation where two or more traders or market participants agree to coordinate their trading activities to influence market prices or conditions. This can include:

1. *Price fixing*: Agreeing to buy or sell at a specific price to manipulate the market price.

2. *Output restriction*: Agreeing to limit the quantity of trades to influence market prices.

3. *Bid rigging*: Agreeing to submit bids at a specific price to manipulate the auction process.

4. *Wash sales*: Agreeing to buy and sell with each other to create the appearance of market activity.

5. *Matched trades*: Agreeing to buy and sell with each other at a specific price to manipulate market prices.

Collusion can be detected through:

1. *Trade surveillance*: Monitoring trading activity for suspicious patterns.

2. *Communication monitoring*: Monitoring emails, chats, and phone calls for evidence of collusion.

3. *Market monitoring*: Analyzing market data for unusual activity.

4. *Network analysis*: Identifying connections between traders and accounts.

5. *Whistleblower tips*: Receiving tips from individuals with knowledge of collusion.

Regulators and exchanges use these methods to detect and prevent collusion, maintaining fair and orderly markets. Penalties for collusion can include fines, suspension, or even criminal prosecution.