Final answer:
A prefix that indicates 'half' is 'semi-'. It can modify different words to imply 'half' or 'partially', such as in 'semiannual', which means occurring twice a year.
Step-by-step explanation:
A prefix that means half is semi-. In English, a prefix is a group of letters placed before the root of a word to modify its meaning. For example, if we consider the fraction one half, we know that half of one-half is one-quarter. Similarly, when we add the prefix semi- to a word, it implies that it is 'half' or 'partially' the concept of the root word. For instance, semiannual means occurring twice a year, or every half year. This understanding of prefixes helps in deciphering the meaning of new words and in enriching one's vocabulary.