* Prokaryote: A single-celled organism that lacks a membrane-bound nucleus and other internal organelles.
* Protoplasm: The living substance of a cell, including both the cytoplasm and the nucleus.
* Prophase: The first stage of mitosis and meiosis, during which the chromosomes condense and become visible.
* Progesterone: A female sex hormone involved in pregnancy and the menstrual cycle.
* Prognosis: A prediction of the likely course of a disease or illness.
* Prokaryotic: Relating to or characteristic of prokaryotes.
* Proliferation: Rapid increase in the number of cells or organisms.
* Protonema: The thread-like stage in the life cycle of some bryophytes (e.g., mosses).
* Proton: A subatomic particle with a positive electric charge.
* Proenzyme: An inactive precursor of an enzyme.
* Prohibit: To prevent or forbid something.
The prefix "pro" can mean "before," "in front of," "for," or "forward." In the context of science, it often indicates a precursor, a beginning stage, or something that is involved in promoting or facilitating a process.