"In medicine, a tincture is an alcoholic extract (e.g. of leaves or other plant material) or solution of a non-volatile substance; (e.g. of iodine, mercurochrome). To qualify as a tincture, the alcoholic extract is to have an ethanol percentage of at least 40-60% (sometimes a 90% percent pure liquid is even achieved).
"In chemistry, a tincture is a solution that has alcohol as the solvent."
The alcohol is the reason that iodine burns when applied to a wound using the glass applicator rod. My memory from childhood is that the iodine and mercurochrome came in amber glass vials.