Final answer:
Secondary endosymbiosis involves the incorporation of prokaryotes into eukaryotes and is an important mechanism in the evolution of eukaryotes.
Step-by-step explanation:
The phrase that could be used in a description of secondary endosymbiosis is 'Incorporation of prokaryotes into eukaryotes'. Secondary endosymbiosis refers to a process where one cell engulfs another cell, which had already undergone primary endosymbiosis. This incorporation of prokaryotes into eukaryotes has led to the evolution of organelles such as mitochondria and chloroplasts within eukaryotic cells. This phenomenon supports the endosymbiotic theory and is an important mechanism in the evolution of eukaryotes.