Who, and in what context, was the 19th / 20th century physicist who
first coined the term 'relativity'?

Hello.

Several published sources credit Henri Poincaré, a French physicist
and mathematician, with first coining the term "relativity."

See:

"The term relativity, coined not by Einstein but by Poincaré and
applied to the theory by the physicist Max Planck..."
source:
"Coming of Age in the Milky Way"
by TIMOTHY FERRIS, read using the "search inside" feature on Amazon.com
http://www.amazon.com/exec/obidos/tg/detail/-/0385263260/

'Henri Poincaré (who was responsible for the term "relativity")'
source:
"A New View of the Universe" by Ben Kristoffen, University of
California at Santa Barbara
http://arxiv.org/pdf/physics/9612010
(This document is in PDF format, so the Adobe Acrobat Reader is
required. If you don't have that, visit:
http://www.adobe.com/products/acrobat/readstep2.html )


Here is the context of Poincare's usage of the term:

'In a widely heralded address made at the St. Louis exhibition in
1904, he announced "...the principle of relativity, according to which
the laws of physical phenomena should be the same, whether for an
observer fixed, or for an observer carried along in a uniform movement
of translation; so that we have not and could not have any means of
discerning whether or not we are carried along in such a motion."'
source: THE POSTULATES OF THE SPECIAL THEORY OF RELATIVITY, University of Dallas
http://phys.udallas.edu/C3P/timeline/RELPOSTS.PDF

Poincaré's lecture was published in 1905. For the complete text, see:
THE PRINCIPLES OF MATHEMATICAL PHYSICS.
by Henri Poincare, hosted thesciencebookstore.com
http://www.thesciencebookstore.com/etext/poincarephysics.html

As indicated, Poincaré talks about the "principle of relativity" in
the context of a general discussion of the principles the govern the
physical operation of the universe. Poincaré lists the "principle of
relativity" along with other general principle such as Newton's
principle of the equality of action and reaction and Lavoisier's
principle of the conservation of mass.

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search strategy:
"the term relavity"
"the term relavity" poincare
amazon.com "term relavity" coined

I hope this helps.

Thank you very much - speedy response and very well referenced answer.