An individual "accredited investor" is either a natural person who has individual net worth, or joint net worth with the person's spouse, that exceeds $1 million at the time of the purchase, excluding the personal residence, OR a natural person with income exceeding $200,000 in each of the two most recent years or joint income with a spouse exceeding $300,000 for those years and a reasonable expectation of the same income level in the current year. The amounts and rule come from Rule 501 of Regulation D.
For the complete definition of accredited investor, see the SEC website.