also: つばき
noun
General term for saliva or spit. Can be used in everyday conversation.
緊張して唾が飲み込めなかった。
I was so nervous I couldn't swallow my saliva.
道に唾を吐いてはいけません。
Don't spit on the street.
Kana spelling is common in everyday writing.
唾液 is a more formal, medical term for saliva, while 唾 is the common, everyday word.
The origin is uncertain, but it is a native Japanese word. The kanji 唾 is conventionally used for this meaning.