anything that comes off the surface:
of liquids, foam, froth, in Hom. of the sea, Od. 12.238, al.; ἁλὸς ἄ. 5.403, cf. Tim. Pers. 95, A.R. 2.570; θοὴν ἀπερεύγεται ἄχνην, of a river, D.P. 693: Medic., exudation, Hp. Int. 1; οἰνωπὸς ἄ. froth of wine, E. Or. 115; ἄχνα οὐρανία dew of heaven, S. OC 681 (lyr.); δακρύων ἄχνα dewy tears, Id. Tr. 848 (lyr.); also ἄχνη πυρός, i.e. smoke, A. Fr. 336.
of solids, chaff, in pl., ὡς δʼ ἄνεμος ἄχνας φορέει Il. 5.499; καρπόν τε καὶ ἄχνας ib. 501; down on the quince, μῆλον λεπτῇ πεποκωμένον ἄ. AP 6.102 (Phil.); ἄχνη ἡ ἀφʼ ἡμιτυβίου fluff, shreds, used for lint, Hp. Art. 37; ὀθονίου Id. Mochl. 2; ἄ. Λυδῆς κερκίδος, of finespun fabrics, S. Fr. 45; ἄ. χαλκίτιδος metallic dust, Plu. QConv. 2.659c, cf. Orph. L. 455; ἄχναι wall-decorations, dub. in Aret. CA 1.1 (stramina Cael. Aur.).
ἄχνην in acc., as Adv., morsel, the least bit, ἢν δʼ οὖν καταμύσῃ κἂν ἄχνην Ar. V. 92.
πυρὸς ἄ., = χαμελαία, Dsc. 4.171; = θυμελαία, ib. 172.
ἄχναν· τὴν οἴκησιν, Hsch.