Hap or HAP can mean:

• Hap Arnold (1886-1950), World War II American Air Force general
• Hap Collard (1898-1968), Major League Baseball pitcher
• Hap Day (1901-1990), National Hockey League (NHL) player, coach, and general manager, member of the Hockey Hall of Fame
• Hap Emms (1905-1988), NHL player, coach, general manager and team owner
• Hap Farber (born 1948), American football player
• Harrison Farber, Professor of Medicine and Director of the Pulmonary Hypertension Center at Boston University
• Frank S. Farley (1901-1977), American politician
• Hobart R. Gay (1894-1983), US Army lieutenant general
• Hap Hadley (1895-1976), American artist
• Hap Holmes (1892-1941), NHL goaltender
• Emil Huhn (1892-1925), American Major League Baseball player
• Louis Kuehn (1901-1981), American diver and 1920 Olympic champion
• Harry McSween (born 1945), Professor of Planetary Geoscience and Distinguished Professor of Science at the University of Tennessee at Knoxville
• Hap Marre, American soccer player of the 1910s
• Herb Mitchell (ice hockey) (1895-1969), Canadian NHL player
• Hap Moran (1901-1994), National Football League halfback
• Hap Kliban (1935-1990), American cartoonist
• Hap Myers (1887-1967), Major League Baseball first baseman
• Hap Myers (ice hockey) (born 1947), NHL defenceman
• Hap Palmer (born 1942), American children's musician
• Harold Ridley (Jesuit) (1939-2005), Roman Catholic priest and President of Loyola College in Maryland
• Hap Sharp (1928-1993), American race car driver
• Henry Augustus Pearson Torrey (1837–1902), professor of philosophy
• Khairul Fahmi Che Mat (born 1989), Malaysian goalkeeper

Hydroxylapatite, also called hydroxyapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca₅(PO₄)₃(OH), but is usually written Ca₁₀(PO₄)₆(OH)₂ to denote that the crystal unit cell comprises two entities. Hydroxylapatite is the hydroxyl endmember of the complex apatite group. The OH^- ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxylapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.