To run across something means to encounter or find it unexpectedly, often by chance rather than through deliberate searching. You might run across an old friend at the grocery store, or run across a fascinating book while browsing the library shelves.

The phrase suggests a pleasant surprise or fortunate discovery. When a researcher runs across an important clue in old documents, she wasn't specifically looking for it but stumbled upon it while reading. When you run across a great recipe online, you found it by accident while looking at something else.

The phrase works with both people and things. You might run across your neighbor at the park, run across a typo while proofreading, or run across a rare coin in a drawer. The key idea is that the encounter wasn't planned: you weren't searching for that specific thing, but there it was.

Similar phrases include “come across,” “stumble upon,” and “happen upon.” All suggest the element of chance that makes running across something different from deliberately seeking it out.