A well-formed formula 
is said to be true for the interpretation 
(written 
)
iff every sequence in 
(the set of all denumerable sequences of elements of the
domain of 
), satisfies 
. 
is said to be false for 
iff no sequence in 
satisfies 
.
Then an interpretation 
is said to be a model for a set 
of well-formed formulas iff every well-formed formula in
is true for 
(Mendelson 1997, pp. 59-60).
