The United Nations Children’s Fund (UNICEF) says child marriage is the formal or informal union where one or both people are younger than 18 years old.
It said child marriage can be associated with lack of access to education and some children being forced to work for their parents-in-law.
Advocacy group, Girls Not Brides, said the vast majority of children affected were girls and child brides were more at risk of contracting HIV/AIDS and more at risk of domestic abuse.
In some countries girls are married before their teens.
Not all child or early marriage is harmful, and marriage before 18 years of age is allowed sometimes.
In Australia, early marriage is legal if a court gives approval and one of the engaged is an adult.
In some cases, early marriage may be preferable in Australia, Australian Christian Lobby managing director Lyle Shelton said.
He said the UN was right to push for stronger laws against child marriage, however, teenage pregnancy in Australia does occur.
"A court can grant an order allowing a 16 year-old to marry and this may be appropriate in certain circumstances depending on the individual," Mr Shelton said.
Globally, more than 700 million women alive today were married before their 18th birthday, UNICEF said.
Jennifer Burn is Anti-Slavery Australia’s director and an Associate Professor at the University of Technology, Sydney, and she specialises in immigration and administrative law.
She said data on girls and women between 15 and 19 years was a good indicator, however, there were married girls younger than 10 years old.
Prof Burn said the United Nations’ resolution was a good guide for what laws should be.
“Each of the states would have to incorporate that into legislation,” Prof Burn said.
Many parents will seek to have a child married to secure the child's financial future, Prof Burn said.
Religion, traditions and perceptions for the roles of women are also factors.
Child marriage was a human rights issue, Prof Burn said.
“Underage marriage can be hugely detrimental to a child,” Prof Burn said.
In Australia, people younger than 18 years of age may not marry unless their partner is an adult and they have proof of "exceptional circumstances".
Girls Not Brides said the United Nations' resolution was a positive step towards ending child marriage.
"The passage of a UN resolution does not mean that we will end child marriage tomorrow, but resolutions are important in setting global norms," Girls Not Brides Global Coordinator Heather B. Hamilton said.
