DIA or Dia may refer to:

• Defense Intelligence Agency, United States foreign military intelligence agency
• Department of Internal Affairs (New Zealand)
• Department of Indian Affairs, Canadian federal department
• Defence Intelligence Agency (India)
• Defence Intelligence Agency (Nigeria)
• DIA (South Korea), Korean army defence intelligence agency
• Direzione Investigativa Antimafia, an Italian law-enforcement agency

A day is a unit of time. In common usage, it is an interval equal to 24 hours. It also can mean the consecutive period of time during which the Sun is above the horizon, also known as daytime. The period of time during which the Earth completes one rotation with respect to the Sun is called a solar day.

A day is a unit of temporal measurement for a literal day or epoch of time. Day, DAY or Days may also refer to:

• Daytime (astronomy)
• Awareness day, a date set by a major organisation or government to commemorate a cause of importance on a national or international level

The Defense Intelligence Agency (DIA) is an external intelligence service of the United States specializing in defense and military intelligence. A component of the Department of Defense (DoD) and the United States Intelligence Community (IC), DIA informs national civilian and defense policymakers about the military intentions and capabilities of foreign governments and non-state actors. DIA also ensures intelligence assistance, integration and coordination across uniformed military service intelligence components, which remain structurally separate from DIA. The agency's role is wide-reaching, encompassing collection and analysis of defense-related foreign political, economic, industrial, geographic, and medical and health intelligence. As part of its national IC responsibilities, DIA regularly provides input for the President's Daily Brief.