Definition of "tester"

And I could as hardly spare my gloves as my shirt, or forbeare washing of my hands both in the mornng and rising from the table, or lye in a bed without a testerne and curtaines about it, as of most necessary things.

1603, Michel de Montaigne, translated by John Florio, Essays, volume III, page 13

The half-dozen pieces […] were painted white and carved with festoons of flowers, birds and cupids. […] The bed was the most extravagant piece. Its graceful cane half tester rose high towards the cornice and was so festooned in carved white wood that the effect was positively insecure, as if the great couch were trimmed with icing sugar.

1963, Margery Allingham, chapter 1, in The China Governess‎

With our shaggy jackets drawn about our shoulders, we now passed the Tomahawk from one to the other, till slowly there grew over us a blue hanging tester of smoke, illuminated by the flame of the new-lit lamp.

1851, Herman Melville, chapter 11, in Moby Dick

When after their beaſtly ſport and pleaſure Mounſieur Libid[inoſo] heat of luſt was ſomewhat aſſwaged, and ready to goe, féeling his pocket for a venereall remuneration [i.e., a copper coin] finds nothing but a Teſter, or at leaſt ſo little, that it was not ſufficient to pleaſe dame Pleaſure for her hire. [...] My Ladie would not beléeue Monſ. Libid. a great while, but ſearched and féeled for more coine, [...]

1602, S[amuel] R[owlands], “How a Citizen was Serued by a Curtizan”, in Greenes Ghost Havnting Conie-catchers. […], London: Printed [by Peter Short?] for R[oger] Iackson, and I. North, […]; republished in The Complete Works of Samuel Rowlands: 1598–1628: Now First Collected, volume I, [Glasgow]: Printed [by R. Anderson] for the Hunterian Club, 1880, page 42

A horse-head tester had cutouts for the animal's eyes and ears. A peytral protected the horse's breast, while the crinet, made of overlapping steel plates joined by leather straps or rivets, protected its neck.

2009 January 1, Madeleine Pelner Cosman, Linda Gale Jones, Handbook to Life in the Medieval World, 3-Volume Set, Infobase Publishing, page 275

The tester and crupper, together with the piser, which probably protected the chest, formed the basis of most horse armour […]

2003, Juliet R. V. Barker, The Tournament in England, 1100-1400, Boydell Press, page 175