1.  fthm(n = noun.quantity) fathom - a linear unit of measurement (equal to 6 feet) for water depth;
is a kind of linear measure, linear unit
has parts: pace, yard

2.  fthm(n = noun.quantity) fathom - (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore;
is a kind of capacity measure, capacity unit, cubage unit, cubature unit, cubic content unit, cubic measure, displacement unit, volume unit