“Sunnyside up” refers to eggs cooked so that the yolk is on top and remains runny while the white is cooked on the bottom. Some synonyms for “sunnyside up” are “over easy” (though there is a slight difference as over easy eggs are flipped briefly) and “sunny side”.
“Over easy” eggs are similar in appearance to sunnyside up eggs but are flipped briefly to cook the top of the white a little more. For example, “He ordered his eggs over easy instead of sunnyside up this morning.” This shows that while they are not exactly the same, they have a similar look.
“Sunny side” is a more abbreviated way of saying “sunnyside up”. “She prefers her eggs sunny side rather than fully cooked.” This synonym is a shorter and perhaps more colloquial way of referring to eggs cooked in the same way as sunnyside up.