A view on some of the available definitions for excipients.

An excipient is an inactive substance formulated alongside the active ingredient ("API") of a medication, for the purpose of bulking-up formulations that contain potent active ingredients (thus often referred to as "bulking agents," "fillers," or "diluents"). Bulking up allows convenient and accurate dispensation of a drug substance when producing a dosage form. They also can serve various therapeutic-enhancing purposes, such as facilitating drug absorption or solubility, or other pharmacokinetic considerations.^[1] Excipients can also be useful in the manufacturing process, to aid in the handling of the active substance concerned such as by facilitating powder flowability or non-stick properties, in addition to aidingin vitro stability such as prevention of denaturation over the expected shelf life. The selection of appropriate excipients also depends upon the route of administration and the dosage form, as well as the active ingredient and other factors.

http://en.wikipedia.org/wiki/Excipient

http://www.ema.europa.eu/docs/en_GB/document_library/Scientific_guideline/2009/09/WC500003380.pdf

http://www.usp.org/sites/default/files/usp_pdf/EN/USPNF/chapter3.pdf

Excipients have been defined in many ways, including as inert substances used as vehicles and diluents for drugs. The problem with this definition is that in recent years excipients have proved to be anything but inert, not only possessing the ability to react with other ingredients in the formulation, but also to cause adverse and hypersensitivity reactions in patients. These range from a mild rash to a potentially life-threatening reaction. Different brands of the same drug may contain different excipients, especially preservatives and colourants. The Consumer Medicines Information provides a list of excipients, and information on the safety of individual excipients can be found in drug reference guides. - See more at: http://www.australianprescriber.com/magazine/34/4/112/4#sthash.WhohVxji.dpuf

http://www.australianprescriber.com/magazine/34/4/112/4

http://www.oxforddictionaries.com/definition/english/excipient