"In 1890" means in the specific year 1890, including every date from January 1, 1890 through December 31, 1890. For example, a Wikipedia article opens with the line "The Sherman Antitrust Act is a landmark federal statute in the history of United States antitrust law (or "competition law"), passed by Congress in 1890." It's good enough to know that it was in 1890. They don't bother to tell us that the exact date was July 2nd, 1890.
"In the 1890s" can mean the precise decade that includes every date from January 1, 1890 through December 31, 1899, inclusive.
It can also mean some time in that rough general period.
"In the 1890s, women's fashions were influenced by the bicycle craze." The fashion trends didn't start exactly in 1890 nor end exactly in 1899.