A deal is accretive if pro-forma eps after the transaction exceeds standalone EPS, and dilutive if it does not. For an all-stock-deal the crude rule is that buying a company on a lower pe-ratio than your own is accretive; buying a higher-multiple company is dilutive. For a cash deal it depends on the earnings bought versus the interest or forgone yield on the cash.
Accretion is not the same as value creation. Overpaying for a low-multiple business can raise EPS on day one and still destroy value, which is why management teams lead with the accretion number.
Example: buyer earns $500M on 250M shares, EPS $2.00. It issues 40M shares to buy a target earning $90M. New EPS is $590M / 290M = $2.03, so the deal is 1.5% accretive.
Related: acquisition, all-stock-deal, dilution, exchange-ratio, deal-premium