Two terms M and N are observationally equivalent iff for all contexts C[] where C[M] is a valid term, C[N] is also a valid term with the same value.

In addition suitable contents:
[ al ] [ am ] [ an ] [ ar ] [ at ] [ b ] [ bs ] [ C ] [ co ] [ con ] [ context ] [ er ] [ fo ] [ for ] [ h ] [ id ] [ iff ] [ io ] [ is ] [ it ] [ li ] [ ls ] [ lu ] [ ly ] [ M ] [ ms ] [ N ] [ na ] [ re ] [ sa ] [ sam ] [ se ] [ so ] [ T ] [ text ] [ th ] [ va ] [ value ]