Liquidity
Also: Lightning liquidity, inbound liquidity, outbound liquidity
lightning · intermediate
On Lightning, the available balance on a particular side of a channel that determines whether you can send or receive a given amount.
On Lightning, "liquidity" usually means the directional balance of a channel.
- Outbound liquidity is the BTC on your side of the channel. It's what you can spend through that channel.
- Inbound liquidity is the BTC on your peer's side. It's what you can receive through that channel.
A fresh channel funded entirely by you has 100% outbound liquidity and zero inbound — you can send up to the capacity, but you can't receive anything until some balance shifts to your peer's side. Channels routinely become "exhausted" in one direction; you can drain outbound by sending payments faster than they come back, or accumulate inbound by receiving them.
Acquiring inbound liquidity is the classic new-node problem. Solutions include: paying a service for an inbound channel (LNBig, Voltage), submarine swaps to convert on-chain BTC into channel balance on your peer's side, opening channels to known popular routes so payment flow eventually balances itself, and dual-funded channels (BOLT 1.1+) where both sides contribute funding.
Routing nodes spend a lot of energy managing liquidity across many channels — that's most of what running a routing node actually involves operationally.