Biological dispersal refers to both the movement of individuals (animals, plants, fungi, bacteria, etc.) from their birth site to their breeding site ('natal dispersal') and the movement from one breeding site to another ('breeding dispersal'). The term also encompasses the movement of propagules such as seeds and spores. Technically, dispersal is defined as any movement that has the potential to lead to gene flow. The act of dispersal involves three phases: departure, transfer, and settlement. Each phase is associated with distinct fitness costs and benefits. By simply moving from one habitat patch to another, an individual's dispersal can influence not only its own fitness but also broader processes such as population dynamics, population genetics, and species distribution. Understanding dispersal and its consequences, both for evolutionary strategies at a species level and for processes at an ecosystem level, requires understanding on the type of dispersal, the dispersal range of a given species, and the dispersal mechanisms involved. Biological dispersal can be correlated to population density. The range of variations of a species' location determines the expansion range.

Biological dispersal may be contrasted with geodispersal, which refers to the mixing of previously isolated populations (or entire biotas) following the erosion of geographic barriers to dispersal or gene flow.

Dispersal can be distinguished from animal migration (typically round-trip seasonal movement), although within population genetics, the terms 'migration' and 'dispersal' are often used interchangeably.

Furthermore, biological dispersal is impacted and limited by different environmental and individual conditions. This leads to a wide range of consequences on the organisms present in the environment and their ability to adapt their dispersal methods to that environment.

Some organisms are motile throughout their lives, while others are adapted to move—or be moved—only during specific, limited phases of their life cycles. This stage is commonly referred to as the dispersive phase. The strategies underlying an organism's full life cycle are often shaped by the nature and conditions of this dispersive phase.

In general, there are two basic types:

Due to population density, dispersal may relieve pressure on resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms. Dispersal of organisms is a critical process for understanding both geographic isolation in evolution through gene flow and the broad patterns of current geographic distributions (biogeography).

A distinction is often made between natal dispersal, where an individual (often a juvenile) moves away from the place it was born, and breeding dispersal, where an individual (often an adult) moves away from one breeding location to breed elsewhere.