Definition: the adjective form of systole, referring to the contraction activity of the heart.
Regarding blood pressure, systolic refers to the highest, peak, pressure within the blood stream occurring during each heart beat.
This is as opposed to diastolic blood pressure, which refers to the lowest, nadir, pressure within the blood stream occurring in between heart beats.
Accepted format for blood pressures is to say the systolic (top) pressure over the diastolic (bottom) pressure, e.g 110/70 mmHg (surrounding atmospheric pressure is being used as the reference, i.e. meaning 110 mmHg above atmospheric pressure during the heart contraction and falling the 70 mmHg above atmospheric pressure during heart relaxation).
The ideal blood pressure for an individual is said to be 120/80.