"Short form", also called "plain form" or "informal form".

Short forms are used to quote someone else, in casual conversations (as signs of intimacy), and in making negative requests.

Verbs and い-adjectives in the affirmative - same as their dictionary forms

な-adjectives and noun + です in the affirmative - replace (formal) です - desu with (informal) だ - da

い-adjectives and な-adjectives and noun + ですin the negative - replace (formal) ありません - arimasen with (informal)  ない - nai