Definition of ENDOPLASMIC RETICULUM. : a system of interconnected vesicular and lamellar cytoplasmic membranes that functions especially in the transport of materials within the cell and that is studded with ribosomes in some places — see cell illustration.See endoplasmic reticulum defined for kids. a system of interconnected vesicular and lamellar cytoplasmic membranes that functions especially in the transport of materials within the cell and that is studded with ribosomes in some places — see cell illustration. See endoplasmic reticulum defined for kids.