Plasma membrane is also called as cell membrane.
Cell membrane was discovered by Schwann, but the term was originally used by Nageli and Cramer.
It has many functions and membranes also occur inside the cytoplasm of eukaryotic cells.
It acts as a covering of several cell organelles like nucleus, mitochondria, plastids, lysosomes, Golgi bodies, and peroxisomes etc.
They line endoplasmic reticulum, cover thylakoids in plastids and form cristae inside mitochondria.
Vacuoles are separated from cytoplasm by a membrane called tonoplast.
All external or internal membranes of cell are now called cell membranes or bio membranes. They are quasi-fluid, elastic, pliable and film like thin partitions over and inside cytoplasm with a thickness of about 75 Armstrong.
They are semi permeable for water and selectively permeable or solutes.
There is an electron dense layer or dark layer on either side of middle electron transparent layer.
This trilaminar structure is found in all biological membranes including nuclear reticulum and Golgi bodies.