| dissociation | 1. The act of separating or state of being separated. 2. <chemistry> The separation of a molecule into two or more fragments (atoms, molecules, ions or free radicals) produced by the absorption of light or thermal energy or by solvation. 3. <psychology> A defense mechanism in which a group of mental processes are segregated from the rest of a person's mental activity in order to avoid emotional distress, as in the dissociative disorders or in which an idea or object is segregated from its emotional significance, in the first sense it is roughly equivalent to splitting, in the second, to isolation. 4. A defect of mental integration in which one or more groups of mental processes become separated off from normal consciousness and, thus separated, function as a unitary whole. Origin: L. Sociatio = union (18 Nov 1997) |
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| dissociation by interference | The simultaneous operation of two separate cardiac pacemaking foci that are unassociated because of interference (a normal physiologic phenomenon) due to rendering their respective territories refractory to each other. Usually atrioventricular dissociation is indicated, the rates being quite close to each other with the atrial rate slightly faster than that of the pacemaker in control of the ventricles. Capture is in either direction, usually the ventricle by the atrium, in incomplete dissociation. H Synonym: dissociation by interference. (05 Mar 2000) |
| dissociation constant | <chemistry> In a chemical equilibrium of form A + B = AB, the equilibrium concentrations (strictly, activities) of the reactants are related such that A x B/AB = a constant, Kd, the dissociation constant, that in this simplest case has the dimensions of concentration. When A is H, this is the acid dissociation constant often designated Ka and expressed as pKa (log10Ka). (18 Nov 1997) |
| dissociation constant of a base | Expressed by the general equation [B+][OH-]/[BOH] = Kb, where BOH is the undissociated base. (05 Mar 2000) |
| dissociation constant of an acid | Expressed by general equation [H+][A-]/[HA] = Ka, where HA is the undissociated acid. (05 Mar 2000) |
| dissociation constant of water | Expressed by the equation [H+][OH-] = Kw = 10-14 at 25캜. (05 Mar 2000) |
| dissociation sensibility | The loss of the pain and the thermal senses with preservation of tactile sensibility or vice versa. (05 Mar 2000) |
| incomplete atrioventricular dissociation | A-V dissociation interrupted by ventricular captures. (05 Mar 2000) |
| interference dissociation | The simultaneous operation of two separate cardiac pacemaking foci that are unassociated because of interference (a normal physiologic phenomenon) due to rendering their respective territories refractory to each other. Usually atrioventricular dissociation is indicated, the rates being quite close to each other with the atrial rate slightly faster than that of the pacemaker in control of the ventricles. Capture is in either direction, usually the ventricle by the atrium, in incomplete dissociation. H Synonym: dissociation by interference. (05 Mar 2000) |
| isorhythmic dissociation | <cardiology, physiology> A-V dissociation characterised by equal or closely similar atrial and ventricular rates. (05 Mar 2000) |
| tabetic dissociation | Loss of proprioceptive sensation with retained pain and temperature sensation due to involvement of the posterior columns of the spinal cord. (05 Mar 2000) |
| electromechanical dissociation | Persistence of electrical activity in the heart without associated mechanical contraction; often a sign of cardiac rupture. (05 Mar 2000) |
| light-near dissociation | A stronger near pupil response than light response; due to weak pupillomotor input, Argyll Robertson pupil, dorsal midbrain syndrome, or to misdirection of ciliary muscle fibres into the iris sphincter. Synonym: light-near dissociation. (05 Mar 2000) |
| longitudinal dissociation | Dissociation between parallel chambers of the heart, as between one atrium and the other or between one ventricle and the other, in contrast to dissociation between atria and ventricles. (05 Mar 2000) |
| Abbe theory of image formation | <optics, physics> Abbe's theory is based on the fact that a non-self-luminous particle, which is illuminated by an extraneous source, gives rise to diffracted light rays, in addition to the dioptric pencil. He stated that to form a good microscopical image as many of the diffracted rays as possible should be intercepted by the objective. With closely ruled lines, his theory is easily demonstrated by observing the back lens of the objective, for here the diffracted rays can be observed directly if the aperture diaphragm is closed. It can be shown that, when the illumination is arranged to exclude the diffracted images, resolution is lost. (11 Mar 1998) |