Definition: A type of organic compound made up of four connected rings, which plays an important role in biological processes like carrying oxygen in blood and photosynthesis in plants.

Example: Chlorophyll is a well-known porphyrin that helps plants absorb sunlight for photosynthesis.

Definition: Describing a type of rock that has large crystals embedded in a finer-grained background.

Example: The geologist examined the porphyritic rock to understand its formation process.

Definition: A purple pigment found in the eyes of some freshwater fish and certain frogs, which is similar to another pigment called rhodopsin.

Example: Porphyropsin helps freshwater fish see in low light conditions.

Definition: A type of rock that has large visible crystals, usually feldspar, set in a finer-grained background.

Example: The ancient Romans used porphyry for their sculptures and buildings due to its durability and beauty.

Definition: To indicate or predict something that will happen in the future, often something bad or significant.

Example: The darkening skies seemed to portend a heavy rainstorm later in the day.

Definition: Related to the location or arrangement of something.

Example: The positional changes in the game can greatly affect the outcome.

Definition: A tiny particle that is similar to an electron but has a positive charge instead of a negative charge.

Example: In particle physics, a positron is often produced in certain types of radioactive decay.

Definition: A temporary combination of an electron and its antimatter counterpart, a positron, that behaves similarly to a hydrogen atom.

Example: Scientists study positronium to understand the interactions between matter and antimatter.

Definition: The study of how much medicine or drugs should be given to patients.

Example: Doctors use posology to determine the correct dosage for their patients.

Definition: Located behind or at the back of the anus.

Example: The postanal region is important for certain medical examinations.