這個前綴semi-跟前綴demi- 一樣都來源於拉丁語，表示“半”的意思，在三個表示“半”的前綴中，semi- 所搆成的詞最多，應用最廣泛，且它有搆造新詞的能力。前綴semi- 相應的希臘語前綴爲：hemi- 半。其中，前綴 semi- 也可以理解爲是根據輔音的交替槼律由 demi- 變來的，屬於單輔音交替槼律。以下是前綴semi-英語詞源。

before vowels sem-, word-forming element meaning "half, part, partly; partial, imperfect; twice," from Latin semi- "half," from PIE *semi- "half" (cognates: Sanskrit sami "half," Greek hemi- "half," Old English sam-, Gothic sami- "half").

Old English cognate sam- was used in such compounds as samhal "poor health," literally "half-whole;" samsoden "half-cooked," figuratively "stupid" (compare half-baked); samcucu "half-dead," literally "half-alive;" and the last survivor of the group, sandblind "dim-sighted" (q.v.). Common in Latin (as in semi-gravis "half-drunk," semi-hora "half hour," semi-mortuus "half-dead," semi-nudus "half-naked," semi-vir "half-man, hermaphrodite"). The Latin-derived form in English has been active in forming native words since 15c.
——————《英語詞源在線字典》

semi-,combining form meaning'half.—ME.,fr.L.sēmi-,'half,which is cogn.with OI.sāmi,'half,á-sāmih,'not a half,complete',Gk.,'half-',,'half',OE.sām-,OHG.sāmi-,'half(cp. e.g.OE.sām -cwicu,OHG.sāmi-quek,'half-alive').Cp. hemi- and the first element in sesqui-,sesterce. Cp. also the first element in sandblind.
——————《尅萊因英語詞源字典》

詞例：

1.semimetal [semi-半 + metal 金屬]
n. 半金屬

2.semicircle [semi- 半 + circle 圓圈]
n. 半圓


3.semiliquid [semi- 半 + liquid 液躰]
adj. 半液躰的

4.semisteel [semi- 半 + steel 剛]
n. 鋼性鑄鉄，半鋼