odo

The root odo comes from Greek hodos, meaning 'a road, a path, a way.' To the Greeks a hodos was both a literal road you walked and, by extension, a way of doing or going. Because the rough breathing 'h' usually drops away in English compounds, the root shows up mostly as -od- or -odo-, hidden inside words that, at first glance, seem to have nothing to do with roads.

Follow the path through its prefixes:

- peri- ('around') + hodos → period. Literally 'a way around' — a journey that comes back to where it started, hence a complete cycle. A period of time is one full loop, like the period of a planet's orbit. The grammatical period (the dot ending a sentence) is the point where the sentence completes its circuit.
- epi- ('upon, in addition') + (eis-, 'into') + hodos → episode. Originally a part 'coming in along the way' of a Greek drama — an extra section inserted into the main action. Today it is one installment of a series, one segment along the longer road of a story.
- ex- ('out') + hodos → exodus. The 'road out' — a mass departure. The book of Exodus in the Bible is literally the story of the Israelites' road out of Egypt.
- ana- ('up') + hodos → anode. The 'way up' — the electrode by which electric current was imagined to travel upward into a device. Its partner cathode is the 'way down' (kata-, 'down').
- hodos + metron ('measure') → odometer. A 'road-measurer' — the instrument that counts how far a vehicle has traveled.

Notice the unifying picture: every odo word is really about a path being traveled — a cycle that loops around (period), a stretch along the journey (episode), a way out (exodus), a way up (anode), or the measuring of the road itself (odometer). Once you see the hidden road, these scattered technical words snap together into one family.