stem of a word; the base part to which inflectional endings attach (used in morphology/grammar).
動詞「食べる」の語幹は「食べ」だ。
The stem of the verb '食べる' is '食べ'.
形容詞「高い」の語幹は「高」になり、そこに語尾が付く。
The stem of the adjective '高い' is '高', and endings attach to it.