Definition
When a miner finds a valid block, it must be transmitted to all other nodes on the network. This propagation is not instantaneous due to network latency, bandwidth limitations, and the time required for nodes to validate the block. Larger blocks take longer to propagate.
Propagation delay is the main cause of orphan/stale blocks: if two miners find blocks near-simultaneously, nodes closer to each miner will see different blocks first. Technologies like compact block relay (BIP 152) significantly reduce propagation times by transmitting only transaction IDs rather than full transaction data.
In Simple Terms
The time for a new block to spread across the network. Faster propagation reduces competing blocks.
The time it takes for a newly mined block to reach all nodes across the Bitcoin network. Faster propagation reduces orphan block rates.