A:
* Atom: The basic unit of a chemical element.
* Asteroid: A rocky body orbiting the Sun, smaller than a planet.
* Asexual Reproduction: Reproduction without the involvement of sex cells.
* Atmosphere: The layer of gases surrounding a planet or other celestial body.
Z:
* Zoology: The study of animals.
* Zone: A region or area with specific characteristics.
* Zymogen: An inactive precursor of an enzyme.
* Zooplankton: Tiny animals that drift in water.
Let me know if you'd like more examples or words related to a specific scientific field!