What it is- formal marriage or informal union before age 18.
India is at the 18th spot among the countries following this tradition (47% women married before 18), after Bangladesh(66.2%)and Nepal(51.4%).
Child marriages is prevalent in families of low income who are unable to raise their children.
To bring the truth of this practice, tv shows like Balika vadhu have picked up this issue.
Although in 1929 under the British Rule, the child marriage restraint act was passed which forbade any male below 21 and any female below 18 to get married, it is still a part of the system of the states of Rajasthan, Haryana, Uttar Pradesh to name a few.