There isn't a super common science word that starts with "z", but here are a few options depending on what you're looking for:

General Science:

* Zoology: The study of animals.

* Zymogen: An inactive form of an enzyme.

Biology:

* Zygote: A fertilized egg cell.

* Zooplankton: Microscopic animals that drift in water.

Chemistry:

* Zeolites: A type of mineral used in various applications, like water purification.

Other:

* Zenith: The point in the sky directly overhead.

* Zeroth: The first, and often most important, law of thermodynamics.

Let me know if you had a specific area of science in mind!