Well-defined subcellular structures that are enveloped in membranes are called organelles.

Organelles are essentially the "organs" of a cell, each performing a specific function to keep the cell alive and functioning. Examples of organelles include:

* Nucleus: Contains the cell's genetic material (DNA)

* Mitochondria: Produce energy for the cell through cellular respiration

* Endoplasmic reticulum: Synthesizes proteins and lipids

* Golgi apparatus: Packages and modifies proteins

* Lysosomes: Digest waste materials

* Peroxisomes: Break down fatty acids and detoxify harmful substances

Let me know if you'd like to learn more about any specific organelle!