What’s the difference between a focus and a locus — is it all just hocus-pocus? — and where does nexus fit in?
The technical meaning of focus is “a point of convergence or divergence, or seeming divergence,” in terms of particles of matter. It also refers to adjustment for clear vision as well as the field of clear vision, and it has a geometrical definition of a fixed point. From the scientific sense have developed literal and figurative connotations of a point, center, or area of activity or occurrence, as well as direction, emphasis, perception, and understanding.
The original plural spelling of the word, borrowed directly from Latin and meaning “hearth” (the area of premodern households that was the center of activity), is foci, but focuses is an alternative; the adjectival form is focal. Focus can be employed, depending on context, with or without a preceding article: “This plan lacks focus”; “His story doesn’t have a focus”; “That is the focus of the argument.”
A locus, meanwhile (in Latin, the word means “place”), is a site or location. As is the case with focus, the sense can be figurative or literal. (Unlike focus, locus has only a Latin plural: loci.)
The difference between focus and locus, then, is subtle. Both words can refer to a place where something happens, but the fine distinction is that the former denotes a center of attention and the latter refers to the location itself. A locus may become the focus, but the reverse is not true, because the site preceded the attention focused on it.
Nexus, from the Latin word nectere, meaning “to bind” (the root term is also the basis of connect), can also mean “focus,” but it is better reserved for the senses of “connection” or “link,” or of a connected group or series.
And what of other related – or seemingly related – terms? An axis is a geometrical or physical or figurative line, or a vertebra or a stem. It can also be somewhat synonymous with the terms defined above, relating to a point or a spectrum, or, as in the manner in which it was used in World War II to refer to the alliance between Germany, Italy, and Japan, a partnership. (The Latin word refers to a line or an axle; the plural in both Latin and English is axes.)
A cynosure is a guide or a center of attention. (The word is from Greek by way of Latin and means, literally, “dog’s tail”; it refers to the constellation Ursa Minor, or the Little Bear.) An omphalos (the word is Greek for “navel”) is a focal point or a hub, and a hub (perhaps a variant of hob, a word for a shelf on a hearth) is a center of activity, either on a large scale or at the center of a wheel. An epicenter is, in figurative usage, a center of activity; the original meaning is the point on Earth’s surface above an earthquake’s point of focus.
Now, how about that hocus-pocus? It evidently originated with itinerant performers who incorporated mock-Latin incantations into their magic or juggling acts to impress gullible onlookers, who associated the classical language with learned scholars and ancient mysteries. It is likely a corruption of the blessing “Hoc est corpus meum,” meaning “This is my body.” Hokey-pokey is probably a further variant, but neither it nor hocus-pocus has any focus, locus, or nexus.