A liposome is a small artificial vesicle, spherical in shape, having at least one lipid bilayer. Due to their hydrophobicity and/or hydrophilicity, biocompatibility, particle size and many other properties, liposomes can be used as drug delivery vehicles for administration of pharmaceutical drugs and nutrients, such as lipid nanoparticles in mRNA vaccines, and DNA vaccines. Liposomes can be prepared by disrupting biological membranes (such as by sonication).

Liposomes are most often composed of phospholipids, especially phosphatidylcholine, and cholesterol, but may also include other lipids, such as those found in egg and phosphatidylethanolamine, as long as they are compatible with lipid bilayer structure. A liposome design may employ surface ligands for attaching to desired cells or tissues.

Based on vesicle structure, there are seven main categories for liposomes: multilamellar large (MLV), oligolamellar (OLV), small unilamellar (SUV), medium-sized unilamellar (MUV), large unilamellar (LUV), giant unilamellar (GUV) and multivesicular vesicles (MVV). The major types of liposomes are the multilamellar vesicle (MLV, with several lamellar phase lipid bilayers), the small unilamellar liposome vesicle (SUV, with one lipid bilayer), the large unilamellar vesicle (LUV), and the cochleate vesicle. A less desirable form is multivesicular liposomes in which one vesicle contains one or more smaller vesicles.

Liposomes should not be confused with lysosomes, or with micelles and reverse micelles. In contrast to liposomes, micelles typically contain a monolayer of fatty acids or surfactants.

The word liposome derives from two Greek words: lipo ("fat") and soma ("body"); it is so named because its composition is primarily of phospholipid.^{[citation needed]}

Liposomes were first described by British hematologist Alec Douglas Bangham in 1961 at the Babraham Institute, in Cambridge—findings that were published 1964. The discovery came about when Bangham and R. W. Horne were testing the institute's new electron microscope by adding negative stain to dry phospholipids. The resemblance to the plasmalemma was obvious, and the microscopic pictures provided the first evidence that the cell membrane is a bilayer lipid structure. The following year, Bangham, his colleague Malcolm Standish, and Gerald Weissmann, an American physician, established the integrity of this closed, bilayer structure and its ability to release its contents following detergent treatment (structure-linked latency). During a Cambridge pub discussion with Bangham, Weissmann first named the structures "liposomes" after something which laboratory had been studying, the lysosome: a simple organelle whose structure-linked latency could be disrupted by detergents and streptolysins. Liposomes are readily distinguishable from micelles and hexagonal lipid phases through negative staining transmission electron microscopy.

Bangham, with colleagues Jeff Watkins and Standish, wrote the 1965 paper that effectively launched what would become the liposome "industry." Around that same time, Weissmann joined Bangham at the Babraham. Later, Weissmann, then an emeritus professor at New York University School of Medicine, recalled the two of them sitting in a Cambridge pub, reflecting on the role of lipid sheets in separating the cell interior from its exterior milieu. This insight, they felt, would be to cell function what the discovery of the double helix had been to genetics. As Bangham had been calling his lipid structures "multilamellar smectic mesophases," or sometimes "Banghasomes," Weissmann proposed the more user-friendly term liposome.

A liposome has an aqueous solution core surrounded by a hydrophobic membrane, in the form of a lipid bilayer; hydrophilic solutes dissolved in the core cannot readily pass through the bilayer. Hydrophobic chemicals associate with the bilayer. This property can be utilized to load liposomes with hydrophobic and/or hydrophilic molecules, a process known as encapsulation. Typically, liposomes are prepared in a solution containing the compound to be trapped, which can either be an aqueous solution for encapsulating hydrophilic compounds like proteins, or solutions in organic solvents mixed with lipids for encapsulating hydrophobic molecules. Encapsulation techniques can be categorized into two types: passive, which relies on the stochastic trapping of molecules during liposome formation, and active, which relies on the presence of charged lipids or transmembrane ion gradients. A crucial parameter to consider is the "encapsulation efficiency," which is defined as the amount of compound present in the liposome solution divided by the total initial amount of compound used during the preparation. In more recent developments, the application of liposomes in single-molecule experiments has introduced the concept of "single entity encapsulation efficiency." This term refers to the probability of a specific liposome containing the required number of copies of the compound.