Readability
Whether continuous text can be comfortably read over a passage, a property of a full block of type rather than of single letterforms.
In practice
Readability is how comfortably a full passage of text can be read over time. It depends less on individual letter shapes than on how the type is set: size, line length, line spacing, contrast and alignment. A legible typeface can still become hard to read in very long lines set tightly in light gray on white. Book designers commonly aim for roughly 45 to 75 characters per line.
Often confused with Legibility
Readability concerns comfortable reading of a whole block of text; legibility concerns whether individual letterforms can be told apart.
Try this
Set one paragraph at three line lengths, short, medium and very long, and read each to compare the effort.