Semi- is a numerical prefix meaning "half". The prefix alone is often used as an abbreviation when the rest of the word (the thing which half of is being described) is clear from context.

Semi or SEMI may refer to:

• Semiconductor Equipment and Materials International (SEMI)
• Semiconductor industry, also known as semi or semis in financial news
• Semi-automatic firearm
• Semi-detached house, a type of housing
• Semi-erection, a partial erection when the penis is enlarged but not fully erect
• Semi-final, of a knockout competition
• Semi-formal, (esp. high school) dance
• Semi Ojeleye (born 1994), American basketball player
• Semiquaver, the time interval in music, which is half the length of a quaver
• Semi-submersible, a watercraft which operates mostly submerged
• Semi-trailer truck (UK: articulated lorry), a truck design of a tractor vehicle pulling semi-trailers
  • Semi-trailer, a trailer with wheels at the rear end only
  • Tractor unit (semi truck engine), the motortruck engine drive unit vehicle that pulls the trailers
    • Tesla Semi, vehicle by Tesla, Inc. ; an electric vehicle based tractor unit that pulls trailers
  • Road train, an extreme version of the semi truck train of trailers
• Goo Semi, member of South Korean girlgroup Cignature