Sierra — NATO Phonetic Alphabet for S

Sierra Nevada and Sierra Leone are just two of the countless real places already carrying this word — S's phonetic-alphabet entry is genuine Spanish for "mountain range," borrowed wholesale rather than invented, the same way Kilo borrows from the metric system. Because it's a real word from an existing, widely spoken language rather than an invented term, most listeners already have a rough sense of how to say it correctly before ever seeing the official phonetic chart.

Its maritime signal flag, raised by itself, reports "my engines are going astern" — a vessel signaling that it is moving in reverse.

Unlike Kilo's scientific-metric-prefix familiarity, Sierra's recognizability comes from geography — it's a common word embedded in real place names on nearly every continent, from the Sierra Nevada in the United States and Spain to countless other ranges that share the name, giving most listeners a mental image to anchor the word to even without formal training. It's also carried into English as a term for mountainous terrain generally, well beyond any specific named range.