Definitions of receptor on the Web:

• a cellular structure that is postulated to exist in order to mediate between a chemical agent that acts on nervous tissue and the physiological ...
• sense organ: an organ having nerve endings (in the skin or viscera or eye or ear or nose or mouth) that respond to stimulation
  wordnetweb.princeton.edu/perl/webwn

• An immune receptor (or immunologic receptor) is a receptor, usually on a cell membrane, which binds to a substance (for example, a cytokine) and ...
  en.wikipedia.org/wiki/Receptor_(immunology)

• In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or cytoplasm of a cell, to which a mobile signaling (or "signal") molecule may attach. ...
  en.wikipedia.org/wiki/Receptor_(biochemistry)

• In a sensory system, a sensory receptor is a sensory nerve ending that recognizes a stimulus in the internal or external environment of an ...
  en.wikipedia.org/wiki/Receptor_(sensory_system)

• A structure on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions ...
  en.wiktionary.org/wiki/receptor

• The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines, especially noradrenaline (norepinephrine) and adrenaline (epinephrine). Although dopamine is a catecholamine, its receptors are in a different category. ...
  en.wikipedia.org/wiki/A_receptors

• a molecule on the surface of a cell that serves as a recognition or binding site for antigens, antibodies or other cellular or immunologic components.
  www.thebody.com/content/treat/art6531.html

• 1) In biochemistry: a specialized molecule in a cell that binds a specific chemical with high specificity and high affinity. ...
  www.nsc.org/ehc/glossar2.html

• A molecule within a cell or on a cell surface to which a substance (such as a hormone or a drug) selectively binds, causing a change in the ...
  www.chemdiv.com/en/Information/glossary/

• A molecule inside or on the surface of a cell that binds to a specific substance and causes a specific physiologic effect in the cell.
  www.upmccancercenters.com/dictionary/r.html

• Ecological entity exposed to a stressor.
  www.waterquality.de/hydrobio.hw/RTERMS.HTM

• A specific molecule of a cell that recognizes and binds with other specific molecules, such as hormones.
  www.arimidex.com/glossary/index.aspx

• A specialized molecule that receives information from the environment and conveys it to other parts of the cell; the information is transmitted by a specific chemical that must fit the receptor, like a key in a lock.
  publications.nigms.nih.gov/medbydesign/glossary.html

• Protein on or protruding from the cell surface to which select chemicals can bind. The opiate receptor in brain cells allows both the natural chemical as well as foreign (opiate) chemicals to bind.
  www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossR.html