In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetization and chemical composition. A simple description is that a phase is a region of material that is chemically uniform, physically distinct, and (often) mechanically separable. In a system consisting of ice and water in a glass jar, the ice cubes are one phase, the water is a second phase, and the humid air over the water is a third phase. The glass of the jar is another separate phase. (See State of Matter#Glass)

The term phase is sometimes used as a synonym for state of matter. Also, the term phase is sometimes used to refer to a set of equilibrium states demarcated in terms of state variables such as pressure and temperature by a phase boundary on a phase diagram. Because phase boundaries relate to changes in the organization of matter, such as a change from liquid to solid or a more subtle change from one crystal structure to another, this latter usage is similar to the use of "phase" as a synonym for state of matter. However, the state of matter and phase diagram usages are not commensurate with the formal definition given above and the intended meaning must be determined in part from the context in which the term is used.

Phase or phases may refer to:

William Sanford "Bill" Nye (born November 27, 1955), popularly known as Bill Nye the Science Guy, is an American science educator, comedian, television host, actor, mechanical engineer, and scientist. He is best known as the host of the Disney/PBS children's science show Bill Nye the Science Guy (1993–1998) and for his many subsequent appearances in popular media as a science educator.

William Sanford Nye was born in Washington, D.C., the son of Jacqueline (née Jenkins; c. 1920–2000), a codebreaker during World War II, and Edwin Darby "Ned" Nye (died 1997), also a World War II veteran whose experience in a Japanese prisoner of war camp led him to become a sundial enthusiast. Nye is a fourth-generation Washington, D.C. resident on his father's side of the family. After attending Lafayette Elementary and Alice Deal Junior High in the city, he was accepted to the private Sidwell Friends School on a partial scholarship, graduating in 1973. He studied mechanical engineering at Cornell University, where one of his professors was Carl Sagan, and graduated with a Bachelor of Science in 1977. He was awarded an honorary doctorate by The Johns Hopkins University in May 2008. In May 2011, Nye was awarded an Honorary Doctor of Science degree from Willamette University where he was the keynote speaker for that year's commencement exercises.

PHASE 2 (aka Lonny Wood) is one of the most influential and well known New York City aerosol artists. Mostly active in the 1970s, Phase 2 is generally credited with originating the "bubble letter" style of aerosol writing, also known as "softies". He was also influential in the early hip-hop scene.

Phase 2 is from The Bronx, and attended DeWitt Clinton High School along with a number of other early graffiti artists. Many famous graffiti writers of the early 1970s would meet at a doughnut shop across from the school called the Coffee Shop before heading down to the subway station at 149th Street and Grand Concourse to watch tagged trains on the IRT line pass by. Phase 2 was mentored in graffiti by his friend and neighbor Thomas Lee aka Lee 163d!, one of the pioneers of graf writing in The Bronx.

He began writing in late 1971 under the name Phase 2, a moniker which had a rather mundane provenance. As Phase 2 would later recall, "the previous year we'd given this party. We were getting ready to give another one and I said, 'We'll call this one Phase Two.' I don't know why, but I was stuck on the name. It had meaning for me. I started writing 'Phase 2.'"

Subir Sachdev is a Professor of Physics at Harvard University specializing in condensed matter. He is known for his research on quantum phase transitions, and for a textbook on the subject. In particular, he is famous for exploiting a connection between the electronic properties of materials near a quantum phase transition and the quantum theory of black holes.

Sachdev was educated in India before attending MIT and Harvard, where he obtained his Ph.D. degree in theoretical physics. He held professional positions at Bell Labs (1985–1987) and at Yale University (1987–2005), where he was a Professor of Physics, before returning to Harvard. He currently holds visiting positions at various universities.

Juan Maldacena discovered the AdS/CFT correspondence of string theory in 1997. In 2007, Sachdev, along with Christopher Herzog, Pavel Kovtun and Dam Thanh Son first used the AdS/CFT correspondence of string theory to study problems in condensed matter physics such as the description of 2+1 dimensional boson Hubbard model. With Sean Hartnoll and Markus Müller, these methods were extended to apply to a wider class of systems with variable particle densities.