How many days is considered a missed period?

Amenorrhea: Primary amenorrhea is defined as the absence of menses by 13 years of age when there is no visible secondary sexual characteristic development or by 15 years of age in the presence of normal secondary sexual characteristics. A woman who has previously menstruated can develop secondary amenorrhea, which is defined as absence of menstruation for three normal menstrual cycles or 6 months.

It is late: if you don't get it the day it is anticipated. It might take a while to figure out if it has been completely missed.

Semantics: If you are regular, and a few days go by, you can state that your period is late or "missed" its normal date. By definition, if the whole cycle of normal days go by, then your period was missed in its entirety.