Transdermal:   A route of administration in which drug is applied to the outer layer of the skin.  (This term is often used synonymously with topical administration.)  Drug is absorbed by passing through the skin.  Usually, absorption is slow and therefore drug levels can be maintained over a long period.  This can be used to advantage for many disorders that require steady blood levels of drug.  "Patches" and other application methods have been developed to take advantage of  the transdermal administration route.  Obviously, transdermal administration is also ideal for treatment of local dermatologic disorders.  The rate of drug absorption will depend upon the physical characteristics of the drug, the dose, and the vehicle used for the drug.  Transdermal administration can be used for a wide range of therapeutics including pain management, hormone replacement, nausea and cardiovascular conditions.