这个前缀semi-跟前缀demi- 一样都来源于拉丁语，表示“半”的意思，在三个表示“半”的前缀中，semi- 所构成的词最多，应用最广泛，且它有构造新词的能力。前缀semi- 相应的希腊语前缀为：hemi- 半。其中，前缀 semi- 也可以理解为是根据辅音的交替规律由 demi- 变来的，属于单辅音交替规律。以下是前缀semi-英语词源。

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.
——————《英语词源在线字典》

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.
——————《克莱因英语词源字典》

词例：

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

2.semicircle [semi- 半 + circle 圆圈]
n. 半圆


3.semiliquid [semi- 半 + liquid 液体]
adj. 半液体的

4.semisteel [semi- 半 + steel 刚]
n. 钢性铸铁，半钢