Tetra Pak is a multinational food packaging and processing company of Swedish origin with head offices in Lund, Sweden and Lausanne, Switzerland. The company offers packaging solutions, filling machines and processing solutions for dairy, beverages, cheese, ice-cream and prepared food, including distribution tools like accumulators, cap applicators, conveyors, crate packers, film wrappers, line controllers and straw applicators. Tetra Pak was founded by Ruben Rausing in 1951 as a subsidiary to Åkerlund & Rausing and built on an innovation by Erik Wallenberg, the tetrahedron package, from which the company name was derived. Tetra Pak’s big commercial breakthrough came in the 1960s and 1970s, with the development of the Tetra Brik package and the aseptic packaging technology, which made possible a cold chain supply, substantially facilitating distribution and storage. From the beginning of the 1950s to the mid 1990s the company was headed by the two sons of Ruben Rausing, Hans and Gad, who literally took the company from a family business of 6 employees, in 1954, to a multinational corporation. Tetra Pak is currently the largest food packaging company in the world by sales, operating in more than 170 countries and with over 22,000 employees (2011). The company is privately owned by the family of Gad Rausing through the Swiss based holding company Tetra Laval, which also includes the dairy farming equipment producer DeLaval and the PET bottle manufacturer Sidel. In November 2011, the Tetra Brik carton package was represented at the exhibition Hidden Heroes – The Genius of Everyday Things at the London Science Museum/Vitra Design Museum, celebrating “the miniature marvels we couldn’t live without”. The aseptic packaging technology has been called the most important food packaging innovation of the 20th Century by the Institute of Food Technologists and the Royal Swedish Academy of Engineering Sciences called the Tetra Pak packaging system one of Sweden’s most successful inventions of all time.

Tetra are species of small freshwater fish from Africa, Central America and South America belonging to the biological family Characidae and to its former subfamilies Alestiidae (the "African tetras") and Lebiasinidae. The Characidae are distinguished from other fish by the presence of a small adipose fin between the dorsal fin and caudal fin. Many of these, such as the neon tetra (Paracheirodon innesi), are brightly colored and easy to keep in captivity. Consequently, they are extremely popular for home aquaria.

Tetra is no longer a taxonomic, phylogenetic term. It is short for tetragonopterus, the former genus name of this group of fish, which is Greek for "square-finned" (literally, square-winged).

Because of the popularity of tetras in the fishkeeping hobby, many unrelated fish are commonly known as tetras, including species from differing families. Even fish that are vastly different may be called tetras. For example, payara (Hydrolycus scomberoides) is occasionally known as the "sabretooth tetra" or "vampire tetra".

Pak or PAK may refer to:

Pak armed forces - the Pakistani armed forces: