late 14c., realtif, in grammar, "a relative pronoun," from Old French relatif (13c.), from Late Latin relativus "having reference or relation," from Latin relatus, used as past participle of referre "bring back, bear back" (see refer), from re- "back, again" + lātus "borne, carried" (see oblate (n.)). The meaning "kinsman, kinswoman, person in the same family or connected by blood" is attested from 1650s.

c. 1600, "quality of being very close or compact," from French densité (16c.), from Old French dempsité (13c.), from Latin densitas "thickness," from densus "thick, dense" (see dense). In physics, "the mass of matter per unit of bulk," 1660s.

early 15c., relatif, "having reference (to something), relating, depending upon," from Old French relatif and directly from Late Latin relativus "having reference or relation," from Latin relatus, used as past participle of referre "bring back, bear back" (see refer), from re- "back, again" + lātus "borne, carried" (see oblate (n.)).

Meaning "having mutual relationship, connected with each other" is from 1590s; that of "arising from or determined by relationship to something else" is from 1610s; that of "having or standing in a relation to something else" is from 1650s; that of "not absolute or existing by itself" is by 1704. In grammar, "referring to an antecedent," from 1520s.

"instrument for determining the relative density of solid bodies," by 1858; see pycno- + -meter.

"density, thickness, compactness," mid-15c., from Latin spissitudo "thickness, density," from spissus "thick, dense, compact, close" (source of Italian spesso, Spanish espeso, Old French espes, French épais). Related: Spissated.

a ghost word printed in the 1934 "Webster's New International Dictionary" and defined as a noun used by physicists and chemists, meaning "density." In sorting out and separating abbreviations from words in preparing the dictionary's second edition, a card marked "D or d" meaning "density" somehow migrated from the "abbreviations" stack to the "words" stack. The "D or d" entry ended up being typeset as a word, dord, and defined as a synonym for density. The mistake was discovered in 1939.

Old English þicness "density, viscosity, hardness; depth; anything thick or heavy; darkness; thicket;" see thick + -ness.

"apparatus for ascertaining the comparative density of a solid or liquid," 1848; see dense + -meter.

"unit of magnetic flux density," 1960, from Nikola Tesla (1856-1943), Croatian-born U.S. engineer. Tesla coil is attested from 1896.

"one thing in place of another," 1560s, from Latin, literally "something for something, one thing for another," from nominative (quid) and ablative (quo) neuter singulars of relative pronoun qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns) + pro "for" (see pro-).