18-49 refers to the 18 to 49 year old demographic used by the Nielsen ratings system. Its primary use and meaning is for advertising media planners; alhough Nielsen Media Research maintains statistics on several demographics and cross-sections to establish advertising rates, the 18-49 demographic is most heavily weighted by advertisers because it is considered the age group where brand loyalty is pliable and higher expendable income allows the demographic to purchase luxury products.
For most channels and American programs, the 18-49 demographic is the most meaningful ratings number directly influencing renewal or cancellation. Older-skewing shows such as NCIS and Dancing with the Stars command high total viewership but are less attractive to the network because of its low 18-49 rating. Although Dancing with the Stars is regularly watched by over 10 million people weekly, ABC truncated the show from two airings per week to one weekly airing in Fall 2013 due to low 18-49 ratings. Similarly, despite NCIS being one of the most-watched shows on television, 30 second advertisements during the program cost nearly $100,000 less compared to programs seen by half as many people, including Glee and The Office.
Notable exceptions where the 18-49 demo is not a sole decider of advertising rates include subscription-based networks (HBO, Showtime, Cinemax) and children/teen-oriented networks (Nickelodeon, Cartoon Network, Disney Channel). Specialty networks may also target for a specific demographic; Lifetime, which specifically targets a female audience, commands higher advertising rates for programs with a high 18-34 women rating.