Even if the flesh of an avocado is a fresh and vibrant shade of green when you cut it open, it slowly starts to break down as soon as it makes contact with the air. This is because of polyphenol oxidase in the flesh, an enzyme that makes the avocado brown and turn rancid within hours. If you want to cut an avocado now and eat it later without refrigerating it in between, you have to minimize the oxygen contact that makes the flesh turn brown.
Preserving an Avocado
To slow down the browning process for a cut avocado, you must restrict the contact between its flesh and the air. There are two ways you can do this. The first is to fill a bowl with cold water and submerge the cut avocado. The second is to place the cut avocado in an airtight plastic bag, squeeze out as much air as possible, and seal it. While these processes slow down the browning, they're no substitute for refrigeration, and will only keep your avocado fresh for a few hours. Unless you're planning on finishing the avocado on the same day you cut it, wrap it in plastic and refrigerate it.