Understanding the Stellar Consensus Protocol

But if your collection of participants is a loosely defined network that members can join and leave at will, without needing to coordinate with any central authority, then you need a federated Byzantine agreement system: one that can determine quorums not from some predetermined roster of nodes, but dynamically, from an ever-changing and inevitably incomplete snapshot of membership at a point in time. Federated voting solves this by introducing the novel idea of a quorum slice: a small collection of peers that a node trusts to convey information about the state of voting in the rest of the network. ²
Recall that in a non-federated Byzantine agreement system, a quorum is defined as a majority of all nodes.

Source: medium.com