Several parts combine into one character.

期 is a phonetic-semantic compound: 其 (qí, "this / that" — providing the sound) + 月 (moon / month). The composite meaning derives from the moon component: "one full cycle of the moon's waxing and waning" = a period, a span of time. The character grew from astronomical observation into the abstract sense of "appointed time / expected period / phase." Time as measured by lunar cycles became time as measured by social agreements.

Korean reading "gi." 期間 (gigan, period — universal time-span vocabulary), 時期 (sigi, period / time / juncture), 期限 (gihan, deadline / time limit — used constantly: "submission deadline"), 學期 (hakgi, semester — Korean academic year divides into 1학기 spring and 2학기 fall), 初期 (chogi, early period), 末期 (malgi, end period — used in medical contexts: "terminal patient"), 延期 (yeongi, postponement). Korean academic and project vocabulary leans heavily on 기.

Mandarin qī, 1st tone. 期 (qī), 期间 (qījiān, period), 时期 (shíqī, time period), 学期 (xuéqī, semester), 延期 (yánqī, to postpone). And distinctively, Mandarin uses 星期 (xīngqī, "star-period" = week / weekday): 星期一 Monday, 星期二 Tuesday, etc. The Mandarin word for "week" preserves an astronomical etymology — naming the seven days after planetary cycles, just as Western languages (Saturday = Saturn's day) do. Compare to the alternative 周 (zhōu), also meaning "week" but with different etymology.

Japanese on-readings split. キ (ki) is the standard: 期間 (kikan, period), 時期 (jiki, time), 学期 (gakki, semester), 初期 (shoki, early period), 延期 (enki, postponement). Alternative on-reading ゴ (go) appears in the literary 最期 (saigo, "final period" = one's last moments / death — the moment of dying). Critical homophone caution: 最期 (saigo, final moment / death) and 最後 (saigo, final / last) sound identical but have distinct meanings — 最後 is "last in sequence," 最期 specifically means "death-time / final hours." Japanese funerary and historical writing distinguishes the two carefully.

Memory aid: 其 (sound) + 月 (moon-month) — one cycle of the moon = a period.