From BiblePortal Wikipedia

Hastings' Dictionary of the New Testament [1]

The only instance of this measurement is found in  Acts 27:28, where by successive soundings a depth of 20 and 15 fathoms is obtained. The word employed (ὀργυιά; cf. Herod, ii. 149. 4) denotes the length from finger tip to finger tip of the outstretched arms, measuring across the breast. In tables of length it appears = 4 cubits = 6 feet. The actual measurement thus depends on the length of the cubit or foot. According to recent authorities, the Roman-Attic ft. is given as equivalent to 971 English ft., which yields 70 in. (approximately) as the length of the fathom. This is slightly under our present-day measure of 6 feet. For the fathom of Julian of Ascalon (74. 49 in.) see Encyclopaedia Biblica , article‘Weights and Measures.’

W. Cruickshank.

Webster's Dictionary [2]

(1): ( n.) A measure of length, containing six feet; the space to which a man can extend his arms; - used chiefly in measuring cables, cordage, and the depth of navigable water by soundings.

(2): ( n.) The measure or extant of one's capacity; depth, as of intellect; profundity; reach; penetration.

(3): ( v. t.) To encompass with the arms extended or encircling; to measure by throwing the arms about; to span.

(4): ( v. t.) The measure by a sounding line; especially, to sound the depth of; to penetrate, measure, and comprehend; to get to the bottom of.

Vine's Expository Dictionary of NT Words [3]

1: Ὀργυιά (Strong'S #3712 — Noun Feminine — orguia — org-wee-ah' )

akin to orego, "to stretch," is the length of the outstretched arms, about six feet,  Acts 27:28 (twice).

Smith's Bible Dictionary [4]

Fathom. See Weights and Measures .

Holman Bible Dictionary [5]

 Acts 27:28Weights And Measures

Easton's Bible Dictionary [6]

 Acts 27:28

Hastings' Dictionary of the Bible [7]

Fathom . See Weights and Measures.

Morrish Bible Dictionary [8]

See Weights And Measures

International Standard Bible Encyclopedia [9]

fath´um ( ὀργυιά , ̬orguiá ): The literal meaning is the length of the outstretched arms, and it was regarded as equal to 4 cubits, or about 6 feet. ( Acts 27:28 ). See Weights And Measures .

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [10]

( Ὀργυιά ), a nautical measure of six (Greek) feet in length (strictly 6-81 Engl, feet); properly (as the word implies) the space which one can cover by extending the arms laterally ( Acts 27:28). (See Measure).

The Nuttall Encyclopedia [11]

A measure of 6 ft. used in taking marine soundings, originally an Anglo-Saxon term for the distance stretched by a man's extended arms; is sometimes used in mining operations.