Learn English – Why “horseback riding” and not simply “horse riding”

The word oestrogen comes from the Latin word oestrus, and oesophagus is a Latin word as well.

The oe spelling in Latin originally represented a diphthong [oj] ("oy"), but then later (in Latin) became a long vowel [ee]. When we borrowed such words into English, it was pronounced more like [i], [e], or [ɛ] (depending on the word), following English pronunciation rules.

And since we pronounce them like "estrogen" and "esophagus", the US English spelling was changed to reflect the English pronunciation more closely, by dropping the o. In British English, the connection to the original Latin spelling was retained.

(Something similar to this happened with, e.g., encyclopaedia (British) / encyclopedia (US).)

The words have different but related meanings. A pattern is a general term for anything that has repetition or other regularity, so it can be seen, heard or otherwise perceived. People often talk about patterns on a dress, or patterns in behavior.

A rhythm is a pattern in time, most commonly used to describe music or speech. Another common use of rhythm is when one is said to be "in a rhythm." This means a person is doing something at a sustainable and comfortable pace, almost as if they were following the beat of music.