Definition of "espial"

Secure—unnoted—Conrad's prow pass'd by, / And anchor'd where his ambush meant to lie; / Screen'd from espial by the jutting cape, / That rears on high its rude fantastic shape.

1814, Lord Byron, “Canto I”, in The Corsair, a Tale, London: […] Thomas Davison, […], for John Murray, […], stanza XVII, page 30, lines 597–600

She had, however, the comfort of knowing that her lady's wardrobe was in her power, but it did not avail her much, as the servants of the house were so shocked at the Frenchwoman's refusal to nurse her sick lady, that every eye was upon her in the way of espial and condemnation; […]

1842, [anonymous collaborator of Letitia Elizabeth Landon], chapter XLII, in Lady Anne Granard; or, Keeping up Appearances. […], volume II, London: Henry Colburn, […], page 232

these be most necessarie for the espials belonging vnto a camp

1601, C[aius] Plinius Secundus [i.e., Pliny the Elder], “(please specify |book=I to XXXVII)”, in Philemon Holland, transl., The Historie of the World. Commonly Called, The Naturall Historie of C. Plinius Secundus. […], (please specify |tome=1 or 2), London: […] Adam Islip, published 1635

King. Sweet Gertrude leave us too, For we have closely sent for Hamlet hither, That he, as 'twere by accident, may there Affront Ophelia. Her Father[ ]and my selfe (lawful espials) Will so bestow our selves, that seeing unseene We may of their encounter frankely judge

c. 1599–1602 (date written), William Shakespeare, “The Tragedie of Hamlet, Prince of Denmarke”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act III, scene i], page 265, column 1