quantitative describes anything of, relating to, or expressible in terms of quantity — concerned with how much or how many there is of something, as opposed to what kind of thing it is.
The word does its clearest work inside a method. In chemistry, quantitative analysis determines how much of each constituent a sample contains, where 📝qualitative analysis establishes only which constituents are present at all. In the social sciences the same split names two research traditions: quantitative research collects structured, countable observations from enough cases to support statistical inference and generalization, and it buys that reach by fixing its questions in advance. Whatever the instrument fails to encode as a number does not survive into the analysis.
The term dates to the 1580s, from Medieval Latin quantitativus, formed on Latin quantitas, "quantity," and ultimately on quantus, "of what size, how much" — from the Proto-Indo-European stem kwo- that also supplies the relative and interrogative pronouns. Its earliest English sense was simply "having quantity"; the modern sense of "measurable" is recorded by the 1650s.
Quantification is a form of compression, and its characteristic failure is treating the compressed record as the whole. A number carries an air of precision it may not have earned, and once a metric becomes the objective rather than the reading, it degrades — the observation formalized as 📝Goodhart's Law. The 📝Quantified Self movement is the practice pushed to its personal limit, converting sleep, mood, and movement into series that can be charted, with the same tradeoff: what the tracker cannot count, it cannot show.