To take advantage of or benefit from an opportune moment or situation. Typically used with "an" or "the" preceding "opportunity." After the drudgery of working in an office for five years, Catherine decided to cash in on an opportunity to work as a travel writer for her friend's new tourism agency. Although the market crash left many people with overpriced mortgages, some savvy homeowners cashed in on the opportunity to purchase property at rock-bottom prices.