Si, si, or SI may refer to (all SI unless otherwise stated): Places Mount Si, a mountain in state of Washington. Si River, a river in China Slovenia, a European nation (esp. internet domain suffix and two-letter abbreviation) Solomon Islands, a Pacific island nation Staten Island, a borough of New York City and an island in the Hudson River. Language ISO 639 alpha-2 code for the Sinhalese language Computer science .si, the Internet country code top-level domain for Slovenia Shift In, a… (More on Si)