Endosymbiosis in general is when a cell engulfs another cell and the cell that is engulfed is not broken down and provides function to the engulfing cell. For example, it is thought that eukaryotic cells and organelles such as mitochondria arose from this event.
Secondary symbiosis is when an eukaryotic cell engulfs another eukaryotic cell that has already undergone endosymbiosis. So,
In secondary endosymbiosis, an eukaryotic cell became endosymbiotic in an eukaryotic cell.
