EIGRP stands for Enhanced Interior Gateway Routing Protocol, which is a hybrid routing protocol that uses a successor and a feasible successor to determine the best route to a destination. A successor is the next-hop router that has the lowest cost (or metric) to reach the destination, and a feasible successor is a backup router that has a cost that is less than the cost of the successor. EIGRP uses the successor and the feasible successor to achieve fast convergence and load balancing. EIGRP also uses a Diffusing Update Algorithm (DUAL) to maintain the routing table and avoid routing loops. IS-IS, OSPF, and BGP are other routing protocols that do not use a successor and a feasible successor, but rather use different algorithms and criteria to select the best route.
Reference
1: EIGRP - N10-008 CompTIA Network+ : 2.1
2: CompTIA Network+ N10-008 Certification Study Guide, page 150-151
3: CompTIA Network+ N10-008 Exam Subnetting Quiz, question 20
4: CompTIA Network+ N10-008 Certification Practice Test, question 9