Open defecation is the human practice of defecating outside ("in the open") rather than into a toilet. People may choose fields, bushes, forests, ditches, streets, canals, or other open spaces for defecation. They do so either because they do not have a toilet readily accessible or due to archaic traditional cultural practices. The practice is common where sanitation infrastructure and services are not available. Even if toilets are available, behavior change efforts may still be needed to promote the use of toilets. 'Open defecation free' (ODF) is a term used to describe communities that have shifted to using toilets instead of open defecation. This can happen, for example, after community-led total sanitation programs have been implemented.

Open defecation can pollute the environment and cause health problems and diseases. High levels of open defecation are linked to high child mortality, poor nutrition, poverty, and large disparities between rich and poor. Ending open defecation is an indicator being used to measure progress towards the Sustainable Development Goal Number 6. Extreme poverty and lack of sanitation are statistically linked. Therefore, eliminating open defecation is thought to be an important part of the effort to eliminate poverty.

In 2022, 420 million people (5.25% of the global population) were practicing open defecation, a significant decline from about 1.31 billion (21.42%) in 2000, representing a reduction of 890 million people or 16.17% points over 22 years. Of those practicing open defecation, 275 million (65.6%) were living in just seven countries. In India, for example, the number had decreased by 62% (73% in 2000 to 11% in 2022), showcasing the country's significant efforts to achieve Sustainable Development Goals by 2030. However, despite the progress, India still had the largest number of people practicing open defecation, followed by Nigeria, Ethiopia, Niger, Pakistan and Indonesia.

In ancient times, there were more open spaces and less population pressure on land, open defecation was a common practice which brought fewer health and hygiene problems. With development and urbanization, open defecating started becoming a challenge and thereby an important public health issue, and an issue of human dignity. With the increase in population in smaller areas, such as cities and towns, more attention was given to hygiene and health. As a result, there was an increase in global attention towards reducing the practice of open defecation.

Open defecation perpetuates the vicious cycle of disease and poverty and is widely regarded as an affront to personal dignity. The countries where open defecation is most widely practiced have the highest numbers of deaths of children under the age of five, as well as high levels of undernutrition, high levels of poverty, and large disparities between people of means and the poor.

The term "open defecation" became widely used in the water, sanitation, and hygiene (WASH) sector from about 2008 onwards. This was due to the publications by the Joint Monitoring Programme for Water Supply and Sanitation (JMP) and the UN International Year of Sanitation. The JMP is a joint program by WHO and UNICEF that was earlier tasked to monitor the water and sanitation targets of the Millennium development goals (MDGs); it is now tasked to monitor Sustainable Development Goal Number 6.

For monitoring of the MDG Number 7, two categories were created: 1) improved sanitation and (2) unimproved sanitation. Open defecation falls into the category of unimproved sanitation. This means that people who practice open defecation do not have access to improved sanitation.^{[citation needed]}

In 2013, World Toilet Day was celebrated as an official UN day for the first time. The term "open defecation" was used in high-level speeches, that helped to draw global attention to this issue (for example, in the "call to action" on sanitation issued by the Deputy Secretary-General of the United Nations in March 2013).