What payments must be reported?
A business must file Form 8300 to report cash paid to it if the cash payment is:

Over $10,000,
Received as:
One lump sum of over $10,000,
Two or more related payments that total in excess of $10,000, or
Payments received as part of a single transaction (or two or more related transactions) that cause the total cash received within a 12-month period to total more than $10,000.
Received in the course of trade or business,
Received from the same payer (or agent), and
Received in a single transaction or in two or more related transactions.

What is the definition of a transaction?
A transaction is the underlying event resulting in the transfer of cash. Examples include:

Sale of goods, services or real or intangible property
Rental of goods or real or personal property
Cash exchanged for other cash
Establishment, maintenance of or contribution to a trust or escrow account
A loan repayment
Conversion of cash to a negotiable instrument such as a check or a bond