Definition: a shadow, shade, thick darkness, an outline.
4639 skiá – properly, the shadow of a looming presence; (figuratively) a spiritual reality (good or bad) relating to God's light or spiritual darkness.
a. properly, shadow, i. e. shade caused by the interception of the light: Mark 4:32 (cf. Ezekiel 17:23); Acts 5:15; σκιά θανάτου, shadow of death (like umbra mortis, Ovid. metam. 5, 191, and umbra Erebi, Vergil Aen. 4, 26; 6, 404), 'the densest darkness' (because from of old Hades had been regarded as enveloped in thick darkness), tropically, the thick darkness of error (i. e. spiritual death; see θάνατος, 1): Matthew 4:16; Luke 1:79 (from Isaiah 9:1, where צַלְמָוֶת).
b. a shadow, i. e. an image cast by an object and representing the form of that object: opposed to σῶμα, the thing itself, Colossians 2:17; hence, equivalent to a sketch, outline, adumbration, Hebrews 8:5; opposed to εἰκών, the 'express' likeness, the very image, Hebrews 10:1 (as in Cicero, de off. 3, 17, 69nos veri juris solidam et expresssam effigiem nullam tenemus, umbra et imaginibus utimur).
Apparently a primary word; "shade" or a shadow (literally or figuratively (darkness of error or an adumbration)) -- shadow.