TagSetDelayed

f/:lhs:=rhs assigns rhs to be the delayed value of lhs, and associates the assignment with the symbol f.

Examples

Define an upvalue for a symbol:

f /: g[f[x_]] := x^2
g[f[3]]
(* 9 *)

Associate a rule with a specific symbol:

foo /: bar[foo] := "result"
bar[foo]
(* "result" *)

Please visit the official Wolfram Language Reference for more details.