Compounds having the structure $\ce{RSeR}$ ($\ce{R} \neq \ce{H}$). They are thus selenium analogues of ethers.
Metal salts of selane, $\ce{H2Se}$.
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1364 [Terms] [Paper]