Protocol Revenue
The part of user fees a protocol keeps for itself, rather than passing on to liquidity providers, validators, or other suppliers.
Mechanically it is the output of a fee split: a parameter in the contracts routes a share of every fee to a treasury or a staking contract, and the rest goes to whoever supplied the service. The word revenue is borrowed from company accounting and the differences are important. Nothing is audited; there is no accrual accounting, so the figure is cash-like and lumpy; costs are generally not netted out, in particular token incentives paid to attract the very activity generating the fees; and income denominated in the protocol's own token can be circular. It is also price-dependent, since fees collected in volatile assets are converted at a rate that moves.
In practice
A lending protocol that takes a reserve factor on interest payments records that share as protocol revenue while the rest accrues to depositors.
The common misunderstanding
Protocol revenue is not company revenue: it is unaudited, usually excludes the cost of token incentives, and token holders normally have no legal claim on it.
The figure this maps to
The share of user fees retained by the protocol itself rather than paid out to its service providers.
Limits: There is no accounting standard behind the word revenue here: one provider treats Ethereum's burned base fee as revenue while another treats validator income as revenue and the burn as neither, so the same chain can show figures that differ by an order of magnitude. Treasury inflows frequently…