A measure of the preference of a compound to dissolve in either water or an organic solvent (such as octanol).
More technically, it is the logarithm of the distribution coefficient (D) of a molecule between an aqueous and lipophilic phase, which takes into account the differential distribution of charged and uncharged forms of the molecule.
Charged and non-charged forms of an ionizable compound partition differentially between water and an organic solvent. The distribution of the compound will therefore vary with pH, which alters the proportion of ionized molecules. LogD is therefore used to provide a measure of the lipophilicity of ionizable compounds. For non-ionizable compounds, logD is the same as logP across all pH values.
Antibiotics with activity for Gram-negative bacteria often have a low LogD value.