#nucleus
- Acariobiont — An organism or cellular biological unit structurally lacking a well-defined, individualized cell nucleus. The term is used synonymously with acariote and serves in fundamental biology to describe primitive life forms or cells in early evolutionary stages where genetic material (DNA or RNA) is not enclosed by a nuclear membrane but dispersed directly within the cytoplasm.
- Achromatin — The fundamental substance within the nucleus of a living cell, composed mainly of nucleic acids and proteins, characterized by its inability to bind basic histological dyes during laboratory techniques, remaining transparent; it forms the structural support for chromatin during cell division.
- Amphicaryon — A hybrid, diploid cell nucleus, structurally formed following the process of amphimixis through the complete fusion of two pronuclei (male and female) during egg fertilization. This zygotic nucleus contains the full set of chromosomes inherited from both parents, ensuring the combination and transmission of hereditary genetic material to subsequent generations.
- Apogalacticon — The point in the elliptical orbit of a star, a stellar cluster, or a celestial body orbiting a galaxy's nucleus that is farthest from the galactic center; galactic apoapsis. In stellar dynamics, this point represents the moment when the body's orbital velocity reaches its minimum value due to the maximum distance from the central mass.
- Atom — The smallest particle of a chemical element that retains all the chemical properties of that element, consisting of a central nucleus (containing protons and neutrons) surrounded by a cloud of electrons. In a historical philosophical sense, the term represented the indivisible and indestructible constituent element of the universe.