Yamato kotoba (大和言葉, literally "Japanese words") are native Japanese words, meaning those words in Japanese that have been inherited from Old Japanese, rather than being borrowed at some stage. They are also known as wago (和語). Together with kango (漢語) and gairaigo (外来語), they form one of the three main sources of Japanese words (there is also elaborate Japanese sound symbolism, of mimetic origin).
The word "yamato kotoba" itself is composed of native Japanese words, and hence is an autological word. The synonym wago is instead a kango, and hence a heterological word.
Yamato kotoba form a fundamental part of the Japanese lexicon, similar to native words (from Old English) in English – while borrowed words are used for many technical terms (particularly kango, as with Latin and Greek in English), or for modern or stylish purposes (mostly gairaigo, as with French in English), much of the core vocabulary and commonly used everyday words are of native origin.
As exhibited in the synonyms yamato kotoba/wago, there are often many synonyms from different origins, usually with differences in usage. Very roughly, kango are generally more formal, often restricted to writing, while yamato kotoba are more casual and more often used in speech, but both types of words are commonly used in both speech and writing.
In Japanese names, the family name is generally formed from yamato kotoba, as in 山下 (yama-shita, mountain-bottom), 大岡 (oo-oka, big-hill), and 小林 (ko-bayashi, small/little-woods), while given names are quite diverse.
Yamato kotoba are generally polysyllabic (often three or more syllables), and more closely follow the CV (consonant-vowel, CVCVCV) pattern of Old Japanese. By contrast, kango are often one or two syllables, and more often have terminal consonants, yōon, and long vowels.