LONDON, May 31 (Thomson Reuters Foundation) -

Nearly 46 million people are enslaved around the world, according to the 2016 Global Slavery Index published on Tuesday.

While there is no globally agreed definition of modern slavery, some of the key elements are defined below. Many forms of slavery involve more than one of these elements.

BONDED LABOUR: People become bonded labourers after falling into debt and being forced to work for free to repay the lender. Many will never pay off their loans, and debt can be passed down through the generations. Bonded labour has existed for hundreds of years and flourishes in South Asia in agriculture, brick kilns, mills and factories.

DESCENT-BASED SLAVERY: When people are born into slavery because their families belong to a class or caste of "slaves" in countries that have strict hierarchical social structures.

FORCED LABOUR: When people are forced to work, usually for no or inadequate payment, as a result of violence or intimidation. Many find themselves trapped, often in a foreign country, with their passports confiscated by employers, and unable to leave.

EARLY AND FORCED MARRIAGE: When children, usually girls, under 18 years old are married without their consent and forced into sexual and domestic servitude.

HUMAN TRAFFICKING: This happens when men, women and children are exploited through the use of violence, deception or coercion and forced to work against their will. A key difference from people smuggling is that trafficking is done for the purpose of exploitation. People can be trafficked for many different forms of exploitation such as forced prostitution, forced labour, forced begging, forced criminality, domestic servitude, forced marriage and forced organ removal.

ORGAN TRAFFICKING: Organ removal, while not as prevalent as sex or labour trafficking, is part of a thriving black market run by criminal gangs. It is included in the U.N. Trafficking in Persons Protocol as an exploitative practice.

A survey by Walk Free Foundation, the Australia-based human rights group, increased the estimated number of people in modern slavery to 45.8 million from 35.8 million in 2014.

Here are some key figures from the index:

* Asia is home to an estimated two-thirds of the total number of people living in modern slavery

 * Sub-Saharan Africa accounts for about 15 percent of the world's slaves

* India is the country with the greatest number of people living in some form of modern slavery, estimated at 18.3 million

* China came second with an estimated 3.4 million slaves and Pakistan third with an estimated 2.1 million slaves

* Rounding out the top 10 were Bangladesh, Uzbekistan, North Korea, Russia, Nigeria, Democratic Republic of the Congo and Indonesia

* In terms of concentration, second was Uzbekistan with 3.97 percent of its population in slavery amid reports of forced labour in its cotton industry, then Cambodia with 1.6 percent, India with 1.4 percent and Qatar with 1.36 percent

* The least action was being taken by the governments of North Korea, Iran, Eritrea, Equatorial Guinea, Hong Kong, Central African Republic, Papua New Guinea, Guinea, the Democratic Republic of the Congo, and South Sudan.

SOURCES:

ILO (http://www.ilo.org/), Walk Free Foundation (http://www.walkfreefoundation.org), Free the Slaves (http://www.freetheslaves.net/), Anti-Trafficking Review (http://www.antitraffickingreview.org/index.php/atrjournal)

(Reporting by Belinda Goldsmith, Editing by Ros Russell.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights and climate change. Visit http://news.trust.org)