In the field of biology, the strict definition of cholesterol refers to a molecule that have the following type of structure: four carbons rings attached to each other and to groups of hydrogen, carbon, and oxygen atoms (three rings have six carbons, one rings has five carbons).

The atoms of carbon, oxygen, and hydrogen are attached to each other in very specific ways in cholesterol, and changing the ways of attachment changes the molecule from cholesterol to some other molecule entirely. Cholesterol occurs naturally in our bodies. We call it a lipid because it feels oily and does not readily dissolve in water.

One of the crucial roles cholesterol plays is helping cell membranes stay flexible, and the way it's synthesized in the body is complex (in other words, the cholesterol we eat does NOT directly translate to having high cholesterol in the body).

Cholesterol refers to a particular molecule in the set of molecules called sterols. The word sterols in turn is derived from the word steroid (indicating a certain molecular structure, in this case 4 rings of atoms with a special arrangement) and the chemistry suffix -ol, which indicates that cholesterol is an alcohol (it has an -OH group on one end). [The chole- part is just an additional prefix related to the substance from which cholesterol was first isolated.] Sterols are part of the group of molecules called lipids, which is a general term for biological molecules that can be dissolved in non-polar solvents. (Non-polar means that the electrons are not strongly pulled toward any particular regions of the molecule, so no parts of the molecule have significant electrical charges.) All of which means that cholesterol is basically a type of fat.

Cholesterol is a wax-like compound consisting of 27 carbon atoms, 46 hydrogen atoms, and one oxygen atom. It is present in the cell membranes of all living organisms, but at different quantities: the wax-like properties of the stuff make it a useful stiffener for cell membranes at a variety of temperatures. For example, bacteria and archaea living in hot springs have far more cholesterol in their cell membranes than you or I do. This is because cholesterol has a higher melting temperature than the phospholipids that otherwise make up cell membranes, and so cells with more cholesterol can survive more intense heat. Still, even in multicellular animals (including us), cholesterol makes up about 30% of our cell membranes.

Cholesterol is also used to transport other materials because it asks as a mostly-nonpolar solvent, in contrast to water, which is an extremely polar solvent. Cholesterol bound to proteins and other compounds which are what biochemists are referring to when they refer to "lipoproteins".

Cholesterol is a specific molecule - it has 4 rings of carbon hooked together, and a carbon chain sticking out one end, and an -OH group [a hydroxyl group] at the other end.

I recommend you to read more about on Wikipedia. “Cholesterol is an organic molecule. It is a sterol, a type of lipid. Cholesterol is bio-synthesized by all animal cells and is an essential structural component of animal cell membranes.” Formula:
C₂₇H₄6_O

I once worked in a lab where we fed cholesterol to guinea pigs to try to learn about heart disease.