A<unk>  is a subcategory of a larger group of chemicals known as<unk> s.  An<unk>  is an organic chemical that contains an<unk>  group — an oxygen atom connected to two (substituted) alkyl groups.  A good example of an<unk>  is the solvent diethyl<unk> .
What differentiates a<unk>  from<unk>  types of<unk> s is the substitution (halogenation) of one or more hydrogen atoms with a halogen atom.  Halogen atoms include fluorine, chlorine, bromine, and iodine.  Technically, the element astatine is also a halogen, but due to a number of factors it is rarely, if ever, used in organic chemistry.
Perhaps the most common use of<unk> s has been in anesthesiology.  The first widely used inhalation anesthetic was diethyl<unk> , which is not a<unk> , but it enabled surgeons to perform painful surgeries without the patient being conscious.  Unfortunately for doctors of that era, diethyl<unk>  has the disadvantage of being extremely flammable and in some cases, explosive.  Occasionally, this led to fires or explosions during surgery, and is one of the reasons that diethyl<unk>  is no longer used in hospital settings.  It is still commonly used as a solvent in organic chemistry labs, though with caution.  In some countries, diethyl<unk>  was eventually replaced with non-flammable (but more toxic)<unk>  hydrocarbons such as chloroform and trichloroethane.  Much later, safer<unk>  hydrocarbon anesthetics such as halothane were developed.<unk> s have replaced most<unk>  compounds for use as inhalation anesthetics.<unk> s have the advantages of being non-flammable as well as less toxic than earlier general anesthetics.<unk> s differ from<unk> <unk> s because they contain at least one halogen atom in each molecule.  Examples of<unk> s include the general anethetics isoflurane, desflurane, and sevoflurane.
Currently, all inhalation anesthetics except for halothane are<unk> s, and when used, are always mixed with oxygen or air and then inhaled by the patient before or during surgery.  In most surgical situations,<unk>  drugs such as opiates are used for pain and skeletal muscle relaxants are used to cause temporary paralysis.  Additional drugs such as midazolam may be used to produce amnesia during surgery.
Newer intravenous anesthetics (such as propofol) have increased the options of anesthesiologists, but<unk> s remain a mainstay of general anesthesia.
Halogenated ether
Chloroform
Benzene
Vinyl ether
Nitrazine
Triphenylmethane
Chloroethane
Silane