Final answer:
The Greek prefix for the number seven is 'hepta', as seen in terms like 'heptagon' for a seven-sided polygon and 'heptathlon' for a seven-event athletic competition.
Step-by-step explanation:
The Greek prefix for the number seven is 'hepta'. This prefix can be found in various English words that are derived from Greek, such as 'heptagon', which refers to a seven-sided polygon. Another example is 'heptathlon', an athletic event comprising seven different track and field events. Knowing these Greek prefixes can help in understanding and building a robust English vocabulary.