Telecommunication services are rapidly increasing with the network growth towards the Next Generation Networks (NGN). The main problems of this growth are the extreme difficulty of service management and the introduction of new services. To address these issues we believe that we need to share common understanding of the term Service. We present a literature about service meanings from several communities (Business, Information technology and Telecommunications). Then, based on this review, we propose the NGN Service Definition which is linking the several definitions of services adopted by TMF, ETSI, 3GPP; OMA... This Service Definition is then applied to an example of the real world services: Presence Service.