The definition of fortuitous is happening by chance.
An example of fortuitous is two people who meet during a storm evacuation and wind up marrying each other a few years later.
fortuitous definition by Webster's New World
happening by chance; accidental
bringing, or happening by, good luck; fortunate
Origin: Classical Latin fortuitus ; from forte, by chance ; from fors (gen. fortis), chance, luck ; from Indo-European an unverified form bhtis ; from base an unverified form bher-, to bring from source bear
fortuitous definition by American Heritage Dictionary
Happening by accident or chance. See Synonyms at accidental.
a. Happening by a fortunate accident or chance.
b. Lucky or fortunate.
Origin: Latin fortuītus; see bher-1 in Indo-European roots.
Usage Note: In its best-established sense, fortuitous means “happening by accident or chance.” Thus, a fortuitous meeting may have either fortunate or unfortunate consequences. For decades, however, the word has often been used in reference to happy accidents, as in The company's profits were enhanced as the result of a fortuitous drop in the cost of paper. This use may have arisen because fortuitous resembles both fortunate and felicitous. Whatever its origin, the use is well established in the writing of reputable authors. • The additional use of fortuitous to mean “lucky or fortunate” is more controversial, as in He came to the Giants in June as the result of a fortuitous trade that sent two players back to the Reds. This use dates back at least to the 1920s, when H.W. Fowler labeled it a malapropism, but it is still widely regarded as incorrect.