An Etymological Dictionary of Astronomy and Astrophysics
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The time necessary for the → accretion of a definite amount of mass with a fixed → accretion rate.

→ accretion; → time.

A charge or claim that someone has done something illegal or wrong; the action or process of accusing someone (OxfordDictionaries.com).

→ accuse; → -tion.

A → differential equation that describes the time evolution of the → scalar potential of the field φ. It is expressed by: ∇²φ = (1/c²)∂²φ/∂t², where c is → velocity of → longitudinal waves and ∇² is the → Laplacian operator.

→ acoustic; → wave; → equation.

1) The process or state of acting or of being active.
2) According to → Newton's third law of motion, an external force that is applied to a body and that is counteracted by an equal force in the opposite direction ( → reaction).
3) A quantity whose → dimension (ML²T^-1) coincides with that of → angular momentum, the → impulse of a force, or → energy x → time. The action plays an important part in → analytical mechanics, → quantum mechanics, and in a number of other fields of physics. Initially introduced in analytical mechanics, the concept of action has become a basic ingredient of modern physics, due to the role it has played in the generalization of → variational principle.
4) A scalar quantity computed as a function of the path followed by a system during its evolution between an initial instant t_i and a final instant t_f. It is defined by the → integral of the → Lagrangian between the two instants:
S = ∫L dt
In the framework of the → field theory, the action is expressed by the integral of the → Lagrangian density over the corresponding space-time volume:
S = ∫L_d d⁴x.
In classical physics, the path actually followed by the system is the one for which S is stationary (→ least action problem).
5) → quantum of action.
6) Math.: The action is a → functional, a mathematical relationship which takes an entire path and produces a single number.

Action, from O.Fr. action, from L. actionem, from agere "to do," → act.

Žireš, verbal noun from žir stem of žiridan "to act;" → act. Koneš, noun from kardan "to do, to make," Mid.Pers. kardan, O.Pers./Av. kar- "to do, make, build," Av. kərənaoiti "makes," cf. Skt. kr- "to do, to make," krnoti "makes," karma "act, deed;" PIE base k^wer- "to do, to make."

The instantaneous action of a body on another body independently of the distance separating them. The description of → gravity by → Newton's law and → electrostatics by → Coulomb's law are examples of action at a distance. According to Newton, → gravitation acts directly and instantaneously between two objects. For example, if the Sun should suddenly break apart, the Earth's orbit would be affected instantaneously. However, action at a distance violates the → principle of relativistic causality. According to → general relativity, gravitational effects travel at the → speed of light. For modern physics there is no instantaneous action at a distance.

→ action; → distance.

The time integral associated with the evolution of a physical system in the phase space.

→ action; → variable.

1) The process of inducing or creating a state of → activity.
2) The process of producing a → radioactive isotope by bombarding a → stable → nuclide with → nuclear particles (such as → protons, → neutrons, → alpha particles, heavy ions, etc.).

Verbal noun of → activate; → -tion.

Chemistry: The minimum amount of energy that is required to activate → atoms or → molecules to a condition in which they can undergo a → chemical reaction. Most reactions involving neutral molecules cannot take place at all until they have acquired the energy needed to stretch, bend, or otherwise distort one or more → bonds. In most cases, the activation energy is supplied by → thermal energy.

→ activation; → energy.

An area of the Sun exhibiting → solar activity with the presence of → sunspots, → flares, → faculae, → prominences, and other phenomena associated with intense magnetic fields.

→ active; → region.

The act or process of actualizing.

Verbal noun of → actualize; → -tion.

The act or process of putting into action; activation.

Verbal noun of → actuate.

The ability of the → eye to see separately two points close to each other. It is a measure of the → resolving power of the eye's → optical system and depends on the density of cells in the → retina. The maximum acuity of the normal human eye is around 0.5 minutes of arc.

→ acuity; → vision.

1) The act or process of adapting.
2) The state of being adapted.

→ adapt.

Physiological process whereby the eye adjusts its sensitivity for different levels of illumination.

→ adaptation, → eye.

Same as → adaptation.

→ adaptation.

The operation of combining two or more quantities to obtain a third quantity called their → sum. The result of adding.

Verbal noun of → add.

The → plus sign +. It is believed to be a shortened form of the letters e and t in the L. word et, which, in early German manuscripts was the term for addition. The signs + and - were first used by Johann Wiedmann in 1489.

→ addition; → sign.

1) The act or state of adhering; state of being adhered or united.
2) Steady or devoted attachment, support, etc.; adherence.
3) Physics: The molecular force of attraction in the area of contact between unlike bodies that acts to hold them together (Dictionary.com). See also → cohesion.

Verbal noun from → adhere; → -tion.

The assumption whereby the density fluctuations in the very → early Universe would be produced by compressing or decompressing of all components of a homogeneous Universe. The adiabatic initial conditions lead to coherent oscillations in the form of peaks in the → temperature anisotropy spectrum. See also → acoustic peak, → baryon acoustic oscillation.

→ adiabatic; → initial; → condition.

The act of adopting. The state of being adopted.

→ adopt; → -tion.