Final answer:
Stamina is the ability to read silently for increasingly longer periods of time, showcasing a reader's endurance and ability to engage deeply with the text.
Step-by-step explanation:
The ability to read silently for increasing longer periods of time is known as stamina. This term refers to the endurance one has while engaging in an activity, in this case, reading. It is distinct from fluency, which relates to the ability to read with speed and accuracy, and comprehension, which is the understanding of what is being read. Stamina in reading builds upon the technical and physical proficiencies and is demonstrated when a reader can read closely, annotating, analyzing, and reflecting as they read, thereby reading deeply for understanding.