The term 'stumble upon' can be defined by four crossword solutions: OCCUR (5 letters), HAPPEN (6 letters), FIND (4 letters), and runacross (9 letters). OCCUR means to take place or exist, HAPPEN means to come about unexpectedly, FIND means to come across something by chance, and runacross means to meet or discover something unexpectedly. Together, these words describe the phrase 'stumble upon, ' which is used to describe an unexpected discovery or encounter.