Wiktionary
n. (context cycling English) An organised, mass-participation cycling event, often held annually.
Wikipedia
Cyclosportive is short for the French term randonnée cyclosportive (the spelling cyclosportif is sometimes also used but cyclosportive is correct as randonnée is a feminine noun in French).
Many cyclists use sportives to challenge themselves in a personal battle against the distance and then ultimately, the clock. Some participants in a cyclosportive will ride the event like a race, with prizes awarded and considerable prestige for top place finishers, particularly in events like La Marmotte, L'Étape du Tour and the Ardechoise.
A cyclosportive falls between a traditional cycle road race and the more challenging non-competitive randonnée or Audax events. Riders normally carry a number and the time they take to complete the course is recorded. There is usually an upper time limit within which the course must be completed (unlike many randonnée events, there is no lower limit preventing riders from completing the course quickly). The already lengthy course will traditionally include climbs and difficult riding conditions, adding to the merit of the event (e.g. the cobblestones of the Paris–Roubaix).
The routes will usually be well sign-posted and/or marshalled (some cyclosportives take place on roads which have been closed to motor traffic for the duration of the event), riders will be able to use feeding stations positioned at intervals along the route to replenish their food and drink supplies and mechanical and medical support may also be provided. Some attract thousands of participants - since 2000 l'Etape du Tour has offered places to 8,500 riders each year.