* "endo-" means "within" or "inner". Think of "endocrine" glands which release hormones *within* the body.
* "symbiosis" refers to a close and often long-term interaction between two different species. Think of "symbiotic" relationships where both species benefit.
Now, let's combine these meanings:
* "Endosymbiosis" literally means "living together within". It describes a specific type of symbiotic relationship where one organism lives *inside* another organism.
This understanding helps us grasp the core concept of endosymbiosis: one organism living within another. This is exactly what happened with the evolution of eukaryotic cells. It's believed that mitochondria and chloroplasts (organelles found in eukaryotic cells) were once free-living bacteria that were engulfed by larger cells and eventually became integrated within their hosts.
The word "endosymbiosis" itself encapsulates the essence of this process: a symbiotic relationship where one organism resides *within* the other, contributing to the overall survival and success of both.