There are strict definitions of infertility used by many doctors. However, there are also similar terms, e.g. sub fertility for a more benign condition and fecundity for the natural improbability to conceive. Infertility in a couple can be due to either the woman or the man, not necessarily both.
Reproductive endocrinologists, the doctors specializing in infertility, consider a couple to be infertile if:
The couple has not conceived after 12 months of contraceptive-free intercourse if the female is under the age of 34
The couple has not conceived after 6 months of contraceptive-free intercourse if the female is over the age of 35 (declining .....egg quality of females over the age of 35 account for the age-based discrepancy as when to seek medical intervention)
The female is incapable of carrying a pregnancy to term.