The term 'Either Half of Gemini' refers to the astrological sign Gemini, which is represented by two twins. The crossword solution for this term is 'TWIN, ' which is a four-letter word. Gemini is one of the twelve zodiac signs and is associated with the element of air. It is symbolized by the Twins, Castor and Pollux, who were the sons of Zeus in Greek mythology. People born under the sign of Gemini are said to be adaptable, communicative, and intellectual.