Meerwein's salt, named after Hans Meerwein, are trialkyloxonium salts. Best known (and commercially available) are trimethyloxonium tetrafluoroborate and triethyloxonium tetrafluoroborate.
The compounds are used as exceptionally strong alkylating agents.
Triethyloxonium tetrafluoroborate is most advantageously prepared from boron trifluoride, diethyl ether and epichlorohydrin.  The trimethyloxonium salt is available from dimethyl ether via an analogous route.