Split Horizon is a technique incorporated by distance vector routing protocols for avoiding routing loops by preventing the routing path to be sent/advertised back to the node from which the advertising router has received it.
The split Horizon technique transmits the data packets in forward direction and propagates to all the attached nodes, except that router which sent the new update. This technique prevents routing loops and also sublimates those areas, where Route Poisoning cannot avoid routing loops to occur. This technique is integrated in most of the distance vector routing protocols including RIP, IGRP, EIGRP and VPLS.
Techopedia explains Split Horizon
There are many different paths in an interconnected network and the operational factors are dynamically changing. Routers tend to update their routing table with up-to-date information on the available paths, addresses, broken paths etc. Generally most routing protocols, incorporate a technique/method in which they advertise status updates frequently to their adjacent neighbors. This process however is beneficial but can create serious network bottlenecks if their routing logic is not calculated and will result in routing loops.
Split Horizon is a technique integrated within majority distance vector routing protocols that prevents these routing loops to occur within a network by discarding the address of the source update router, from the list of all routers which it will propagate an update, received from the source router. This technique is similar to route poisoning, which prevents network traffic to flow from a faulty/invalid path.