Quick answer
An optical inter-satellite link (OISL) is a laser link that sends data directly from one satellite to another, so internet traffic can cross a constellation in space without passing through a ground station. Starlink and Amazon Leo both use them.
📘 Full definition
Optical inter-satellite links (OISLs), usually called laser links, connect satellites to each other with infrared laser beams. Each satellite can relay traffic to its neighbours, creating a mesh network in orbit. Because light travels about 47% faster in a vacuum than in glass fibre, long-distance traffic routed through space can beat terrestrial fibre on latency.
Medium
Infrared laser
Amazon Leo Link Speed
Up to 100 Gbps
Starlink
Laser links since 2021 (v1.5)
Speed in a Vacuum
About 47% faster than in fibre
Understanding OISL
Starlink and Amazon Leo
Starlink added laser links with its v1.5 satellites in 2021, and all current Starlink satellites carry them. Amazon Leo has them on every satellite from the first production unit, at up to 100 Gbps per link. Compare both networks on the Starlink vs Amazon Leo page.