An endosome is a small structure inside cells that helps to transport and sort various molecules. It acts like a package delivery center, receiving molecules from outside the cell and sending them to different parts within the cell.
Akhila Chandrashaker (Zerial, MPG)-- «Systems analysis of early endosome motility through identification of molecular motors» (2010) (dresden-ipp.de)
Brunetti, C. R., Dingwell, K. S., Wale, C., Graham, F. L. & Johnson, D. C. Herpes simplex virus gD and virions accumulate in endosomes by mannose 6 - phosphate - dependent and - independent mechanisms. (nature.com)
Also, with some nanoparticles much of the material ends up trapped in protective sacs called endosomes inside the cell, and there can be potential toxic side effects. (news.mit.edu)