Several parts combine into one character.

字 has a beautiful etymology that doubles as a theory of how scripts grow. The compound is 宀 (roof / house) + 子 (child) — the original meaning was "to give birth to and raise children at home". The leap to "character / written sign" came from a metaphor: just as parents produce many children from a single household, the simple base characters (文) produce many derivative characters (字) by combining. So 文 (the original primitive marks) and 字 (the compound characters built from them) form a parent-and-child pair across the entire script.

Identical across 繁體 / 新字体 / 简体.

This distinction between 文 and 字 is real and worth knowing. 文 historically referred to simple pictographs and indicators (日, 月, 山, 木, 上, 下); 字 referred to compound characters built from those primitives (好, 林, 明, 休). When East Asian linguists discuss the script formally, they preserve this distinction.

Mandarin: zì, falling 4th tone. 字 covers "character", "writing", and the unit of one Chinese character: 汉字 (Hànzì, Chinese character — note: this is the standard term; never call them "Chinese letters"), 写字 (xiězì, to write characters), 字典 (zìdiǎn, character dictionary), 数字 (shùzì, numerical figure / digit), 名字 (míngzi, name — neutral final tone). Also a measure word for written units.

Japanese: on-reading ジ (ji) is dominant — 漢字 (kanji, Chinese characters / kanji), 文字 (moji or monji, character / letter), 字幕 (jimaku, subtitles), 数字 (sūji, number/digit), 活字 (katsuji, type / printed character). Kun-reading あざ (aza) survives only as an archaic administrative unit name in rural addresses (~"hamlet").

The word 漢字 (kanji / Hànzì / 한자 hanja) — meaning "characters of the Han people" — is what the entire CJK script system is called when distinguished from the phonetic systems (kana, hangul, pinyin).

Memory aid: a roof above a child — characters are born and raised at home.