now contains the following extensive story: Tait used the symbol for "the very singular operator devised by Hamilton" in An Elementary Treatise on Quaternions (1867, p. 221). Tait made very effective use of the operator in a series of papers, including "On Green's and other allied theorems" (1870) Scientific Papers I, p. 136. The story of its naming is related in Cargill Gilston Knott\x{2019}s Life and Scientific Work of Peter Guthrie Tait (1911): From the resemblance of this inverted delta to an Assyrian harp Robertson Smith suggested the name Nabla. The name was used in playful intercourse between Tait and Clerk Maxwell, who in a letter of uncertain date finished a brief sketch of a particular problem in orthogonal surfaces by the remark "It is neater and perhaps wiser to compose a nablody on this theme which is well suited for this species of composition." [...] It was probably this reluctance on the part of Maxwell to use the term Nabla in serious writings which prevented Tait from introducing the word earlier than he did. The one published use of the word by Maxwell is in the title to his homorous Tyndallic Ode, which is dedicated to the "Chief Musician upon Nabla," that is, Tait. In a letter from Maxwell to Tait on Nov. 7, 1870, Maxwell wrote, "What do you call this? Atled?" In a letter from Maxwell to Tait on Jan. 23, 1871, Maxwell began with, "Still harping on that Nabla?" Maxwell and Tait were school friends and the background to the fun was Maxwell\x{2019}s search for a suitable term to use in his own work. In the event Maxwell called the operator the slope: see A Treatise on Electricity and Magnetism (1873, pp.15-16.) In "On the importance of quaternions in physics" (1890) Scientific Papers II, pp. 303-4 Tait wrote "we must have a name attached to [this remarkable operator] and I shall speak of it as Nabla." Heaviside also used "nabla" but without enthusiasm, "The fictitious vector ... is very important. Physical mathematics is very largely the mathematics of . The name Nabla seems, therefore, ludicrously inefficient." ("On the Forces, Stresses, and Fluxes of Energy in the Electromagnetic Field," Philosophical Transactions of the Royal Society of London. A, 183, (1892), p. 431.) This supersedes most of what is below. ------------------------------------------------------------------------ The most definite information came from Avinoam Mann (MANN@VMS.HUJI.AC.IL) who had posted a contribution to a nabla discussion on the academia mailing list (ACADEMIA@techunix.technion.ac.il), and it was communicated to me by Dani Censor (CENSOR@bguee.ee.bgu.ac.il), the maintainer of the list. Mann refers to two web sites by Jeff Miller, Earliest Known Uses of Some of the Words of Mathematics

Earliest Uses of Various Mathematical Symbols

where one can find the following:
http://members.aol.com/jeff570/mathsym.html
These pages show the names of the individuals who first used various
common mathematical symbols, and the dates the symbols first
appeared. Written sources are listed on a separate page. The most
important written source is the definitive A History of Mathematical
Notations by Florian Cajori.
[Cajori, Florian. A History of Mathematical Notations. 2 volumes.
Lasalle, Illinois: The Open Court Publishing Co., 1928-1929.]
http://members.aol.com/jeff570/calculus.html [from mathsym.html]
The Hamiltonian operator. The symbol , which is also called a "del,"
"nabla," or "atled" (delta spelled backwards), was introduced
by William Rowan Hamilton (1805-1865) in 1853 in Lectures on
Quaternions, according to Cajori vol. 2, page 135.
David Wilkins has found the symbol used earlier by Hamilton in the
Proceedings of the Royal Irish Academy of the meeting held on
July 20, 1846. The volume appeared in 1847. However the symbol is
rotated 90 degrees.
http://members.aol.com/jeff570/m-r.html [from mathword.html]
The word NABLA (for the "del" or Hamiltonian operator) was suggested
humorously by James Clerk Maxwell, according to one source. According to
a post in sci.math by Noam D. Elkies, the term was coined by Tullio
Levi-Civita (1873-1941). A nabla is the name of an Egyptian harp. Cajori
(vol. 2, page 135) says Heaviside called the symbol a nabla.
-----
But Noam D. Elkies (elkies@math.harvard.edu) says he ``cannot
reconstruct the source of the claim about Levi-Civita and nabla'',
and in view of the discussion here, ``while it still makes sense for
L-C to be the coiner, the case is not as strong.''
Two alternative contenders:
Garry Tee mentions that the standard biographies of Kelvin by
S. P. Thompson (1910) and Andrew Gray, and by Crosbie Smith in
"Energy and Empire" (CUP 1989) say something to the effect that
the symbol $\nabla$ was invented (c1870) by William
Thomson (later Baron Kelvin), as a modification of the symbol $\delta$
which he used for the Laplacian operator. The symbol suggests the
shape of a harp, and so Thomson gave it the Greek name.
Michele Benzi refers to p. 143 of
Cargil Gilston Knott, "Life and Scientific Work of Peter Guthrie Tait",
Cambridge, England, 1911.
that nabla was the name suggested to P. G. Tait by Robertson Smith
because of the similarity of the symbol to an Assyrian harp.
-----
As regards language, nabla is the Greek word for some sort of harp.
David Schaps (dschaps@mail.biu.ac.il) points out that the greek word
does not derive from the related Hebrew word nevel=nebel for harp
since it can be found already in the work of Sophocles. But probably
the common origin of both words is aramaic. Indeed, S I Ben-Abraham
(benabr@BGUMAIL.BGU.AC.IL) writes:
I venture to add that

Arnold Neumaier (neum@cma.univie.ac.at)