What is meant by the term "stroke volume" and how can it be modified?

stroke volume is the amount of blood ejected from the ventricles into the arterial bed at each cardiac cycle.

Stroke volume increases with exercise and also in certain disease states like hyperthyroidsm. It depends on the filling of the ventricle and the contractile state of the ventricle, increasing with increase in both. It will be reduced in cardiac failure and hypovolemia.

The two vertical lines are usually much more straight than the horizontal lines. the distance between the two vertical lines is the Stroke volume.-The stroke volume is not all the blood contained in the left ventricle. The heart does not pump all the blood out of the ventricle. Normally only about 2/3 of the blood in the ventricle is put out with each beat. What blood is actually pumped from left ventricle is the STROKE VOLUME and it together with the heart rate determines the cardiac output.
Men on average have higher stroke volumes than women do to larger size hearts. However stroke volume depends on several factors heart size,contractual,duration of contractions.,preload (end diastolic) and after load.
Prolonged aerobic exercise may also increase stroke volume which frequently result in a lower heart rate. Reduce heart rate prolongs ventricular diastole (filling) this allows more blood to be ejected.
Stroke volume is intrinsically controlled by preload (the degree in which the ventricles are stretched prior to contracting) an increase in the speed/volume of venous return will increase preload and through the Frank-Starling law of the heart will increase heart volume. Decreased venous return has the opposite effect causing a reduction in stroke volume.
Elevated after load reduces stroke volume. Through not usally affecting stroke volume in healthy individuals increased after load will hinder the ventricles in ejecting blood causing reduced stroke volume .Increased after load may be found stenosis and arterial hypertension.

Your definition of stroke volume is correct. With exercise the stroke volume increases. Certain cardiac drugs increase while others decrease the stroke volume.
In athletes the stroke volume can be higher than the average person.
In general for the stroke volume to increase the cardiac muscle should be healthier leading to stronger and more complete contractions.
Hence in many diseased conditions of the cardio vascular system the stroke volume tends to get reduced.