H3

H3 is an abbreviation that can refer to several distinct concepts in science and technology. It is commonly used to denote a hydrogen isotope, a markup heading level in web documents, and a hexagon-based geospatial indexing system developed for mapping applications.

Hydrogen-3, or tritium, is a radioactive isotope of hydrogen with a mass number of 3. Its symbol is written as 3H or ^3H. Tritium decays by beta emission to helium-3, with a half-life of about 12.3 years. It occurs naturally in trace amounts and is manufactured for applications such as fusion research, self-luminous devices, and tracing chemical processes. Because of its radioactivity, handling and disposal are regulated.

HTML and other markup languages use the H3 element to indicate a third-level heading. H3 elements hierarchically subdivide content, typically nested under H2 headings. They play a key role in document structure, accessibility, and search-engine understandability, and are often styled through CSS to indicate importance and visual hierarchy.

H3 is also the name of an open-source hexagon-based geospatial indexing system created by Uber Technologies. The H3 library assigns a unique identifier to each hexagon at a given resolution, enabling efficient spatial indexing, aggregation, and querying across large datasets. Resolutions range across multiple levels, allowing scalable analysis of locations and regions on the sphere.