連用形. An inflection of verbs and adjectives known by various names in English, including "continuative form", "infinitive", "the stem", "conjunctive form", and even "Vmasu form" (since it's the form that appears before 〜ます, although of course this name doesn't work for adjectives). Sometimes called 中立形 in Japanese. Formed with -i for consonant-stem verbs, -∅ for vowel-stem verbs, and -ku for adjectives.
Questions about the 連用形{れんようけい} ren'yōkei form of verbs, known by the English terms "continuative" or "conjunctive".