What does the term 'fathom' primarily refer to in a nautical context?

In a nautical context, the term 'fathom' primarily refers to a unit of measurement for marine depth. One fathom is equivalent to six feet and is used to indicate how deep the water is. The term has its origins in Old English, where it referred to the span of a person's outstretched arms, which was approximately this depth. When navigating or setting anchor, sailors rely on fathoms to ensure safe depths and to avoid hazards like shoals or underwater obstacles. Understanding this term is crucial for mariners as it directly relates to the safety and effectiveness of their navigation.