attenuation

from The Century Dictionary.

• noun The act or process of making slender, thin, or lean; the state of being thin; emaciation; reduced thickness or proportions.
• noun The act of making fine by comminution or attrition.
• noun The act or process of lessening in complexity or intensity; reduction of force, strength, or energy; specifically, in homeopathy, the reduction of the active principle of medicines to minute or infinitesimal doses.
• noun The act of making thin or thinner, as a fluid, or the state of being thin or thinned; diminution of density or viscidity: as, the attenuation of the humors; specifically, in brewing and distilling, the thinning or clarifying of saccharine worts by the conversion of the sugar into alcohol and carbonic acid.
• noun In electricity: The decrease of telephonic currents with increasing distance, due to the absorption of current by the electrostatic capacity of the telephone line.
• noun More generally, the decrease of electrical effects with increasing distance.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act or process of making slender, or the state of being slender; emaciation.
• noun The act of attenuating; the act of making thin or less dense, or of rarefying, as fluids or gases.
• noun The process of weakening in intensity; diminution of virulence.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A gradual diminishing in the strength of something.
• noun physics A reduction in the level of some property with distance, especially the amplitude of a wave or the strength of a signal.
• noun biology A weakening in the virulence of a pathogen or other microorganism.
• noun botany The tapering of a leaf etc to a fine point.
• noun engineering A fabrication process in which a material is stretched out into a thin shape.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the property of something that has been weakened or reduced in thickness or density
• noun weakening in force or intensity