Mu (letter) in the context of Muons

A muon (/ˈm(j)uː.ɒn/ M(Y)OO-on; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of ⁠1/2⁠ ħ, but with a much greater mass. It is classified as a lepton. As with other leptons, the muon is not thought to be composed of any simpler particles.

The muon is an unstable subatomic particle with a mean lifetime of 2.2 μs, much longer than many other subatomic particles. As with the decay of the free neutron (with a lifetime around 15 minutes), muon decay is slow (by subatomic standards) because the decay is mediated only by the weak interaction (rather than the more powerful strong interaction or electromagnetic interaction), and because the mass difference between the muon and the set of its decay products is small, providing few kinetic degrees of freedom for decay. Muon decay almost always produces at least three particles, which must include an electron of the same charge as the muon and two types of neutrinos.

United States micropolitan statistical areas (μSA, where the initial Greek letter mu represents "micro-"), as defined by the Office of Management and Budget (OMB), are labor market and statistical areas in the United States centered on an urban cluster (urban area) with a population of at least 10,000 but fewer than 50,000 people. The micropolitan area designation was created in 2003. Like the better-known metropolitan statistical areas, a micropolitan area is a geographic entity used for statistical purposes based on counties and county equivalents. On July 21, 2023, the Office of Management and Budget released revised delineations of the various core-based statistical areas (CBSAs) in the United States, which recognized 542 micropolitan areas in the United States, four of which are in Puerto Rico.

The term "micropolitan" gained currency in the 1990s to describe growing population centers in the United States that are removed from larger cities, in some cases by 100 miles (160 km) or more.

In physics and engineering, mass flow rate is the rate at which mass of a substance changes over time. Its unit is kilogram per second (kg/s) in SI units, and slug per second or pound per second in US customary units. The common symbol is ${\displaystyle {\dot {m}}}$ (pronounced "m-dot"), although sometimes ${\displaystyle \mu }$ (Greek lowercase mu) is used.

Sometimes, mass flow rate as defined here is termed "mass flux" or "mass current".Confusingly, "mass flow" is also a term for mass flux, the rate of mass flow per unit of area.

Micro (Greek letter μ, mu, non-italic) is a unit prefix in the metric system denoting a factor of one millionth (10). It comes from the Greek word μικρός (mikrós), meaning "small".

It is the only SI prefix which uses a character not from the Latin alphabet. In Unicode, the symbol is represented by U+03BC μ GREEK SMALL LETTER MU or the legacy symbol U+00B5 µ MICRO SIGN. The prefix "mc" is also commonly used; for example, "mcg" denotes a microgram (whereas mg denotes a milligram).

In electromagnetism, permeability is the measure of magnetization produced in a material in response to an applied magnetic field. Permeability is typically represented by the (italicized) Greek letter μ. It is the ratio of the magnetic induction ${\displaystyle B}$ to the magnetizing field ${\displaystyle H}$ in a material. The term was coined by William Thomson, 1st Baron Kelvin in 1872, and used alongside permittivity by Oliver Heaviside in 1885. The reciprocal of permeability is magnetic reluctivity.

In SI units, permeability is measured in henries per meter (H/m), or equivalently in newtons per ampere squared (N/A). The permeability constant μ₀, also known as the magnetic constant or the permeability of free space, is the proportionality between magnetic induction and magnetizing force when forming a magnetic field in a classical vacuum.

In coordination chemistry, denticity (from Latin dentis 'tooth') refers to the number of donor groups in a given ligand that bind to the central metal atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be unidentate or monodentate. Ligands with more than one bonded atom are called multidentate or polydentate. The denticity of a ligand is described with the Greek letter κ ('kappa'). For example, κ-EDTA describes an EDTA ligand that coordinates through 6 non-contiguous atoms.

Denticity is different from hapticity because hapticity refers exclusively to ligands where the coordinating atoms are contiguous. In these cases the η ('eta') notation is used. Bridging ligands use the μ ('mu') notation.

The μ-opioid receptors (using the Greek letter mu, abbreviated MOR) are a class of opioid receptors with a high affinity for enkephalins and beta-endorphin, but a low affinity for dynorphins. They are also referred to as μ(mu)-opioid peptide (MOP) receptors. The prototypical μ-opioid receptor agonist is morphine, the primary psychoactive alkaloid in opium and for which the receptor was named, with mu being the first letter of Morpheus, the compound's namesake in the original Greek. It is an inhibitory G-protein coupled receptor that activates the G_i alpha subunit, inhibiting adenylate cyclase activity, lowering cAMP levels.

Mem (also spelled Meem, Meme, or Mim) is the thirteenth letter of the Semitic abjads, including Hebrew mēm מ‎, Aramaic mem 𐡌, Syriac mīm ܡ, Arabic mīm م‎, and Phoenician mēm 𐤌. Its sound value is [m]. It is also related to the Ancient North Arabian 𐪃‎‎‎, South Arabian 𐩣, and Ge'ez መ. The Phoenician letter gave rise to the Greek mu (Μ), Etruscan , Latin M, and Cyrillic М.

In the metric system, a microgram or microgramme is a unit of mass equal to one millionth (1×10) of a gram. Two different abbreviations are commonly used. The International System of Units (SI) uses μg, where the SI prefix "micro-" is represented by the Greek letter μ (mu). The abbreviation mcg is preferred for medical information in the United States (US), but prescription writing guidance in the United Kingdom advises that "microgram" should not be abbreviated. A third abbreviation, the Greek letter γ (gamma), is no longer recommended.The US Institute for Safe Medication Practices (ISMP) and the US Food and Drug Administration (FDA) recommend that mcg should be used, rather than μg, when communicating medical information. This is due to the risk that μ might be misread as m, for "milli-", which is equal to one thousandth (1×10). Such a misreading could result in a thousandfold overdose of a drug or medicine. However, mcg is also the symbol for the obsolete unit millicentigram, derived from the centimetre–gram–second system of units and equal to 10 μg.

One millionth is equal to 0.000 001, or 1 x 10 in scientific notation. It is the reciprocal of a million, and can be also written as 1⁄1,000,000. Units using this fraction can be indicated using the prefix "micro-" from Greek, meaning "small". Numbers of this quantity are expressed in terms of μ (the Greek letter mu).

"Millionth" can also mean the ordinal number that comes after the nine hundred, ninety-nine thousand, nine hundred, ninety-ninth and before the million and first.