"A dimensionless concentration notation which denotes the amount of a given substance in a total amount of 1,000,000 regardless of the units of measure used as long as they are the same or 1 part in 10^[6]." [UOC:GVG]
"A dimensionless concentration notation which describes the amount of one substance in another. It is the ratio of the amount of the substance of interest to the amount of that substance plus the amount of the substance." [Wikipedia:Wikipedia]
Ontology