lightness

light (“illumination, visible light”) +‎ -ness

lightness (countable and uncountable, plural lightnesses)

1. (uncountable) the condition of being illuminated
2. (uncountable) the relative whiteness or transparency of a colour
3. (countable) The product of being illuminated.

light (“having little weight”) +‎ -ness

lightness (uncountable)

1. The state of having little weight, or little force.
2. Agility of movement.
3. Freedom from worry.
   • 1852, Mrs M.A. Thompson, “The Tutor's Daughter”, in Graham's American Monthly Magazine of Literature, Art, and Fashion, page 266:

     In the lightness of my heart I sang catches of songs as my horse gayly bore me along the well-remembered road.

4. Levity, frivolity; inconsistency.
   • 1621, Democritus Junior [pseudonym; Robert Burton], The Anatomy of Melancholy, Oxford: Printed by Iohn Lichfield and Iames Short, for Henry Cripps, OCLC 216894069; The Anatomy of Melancholy: […], 2nd corrected and augmented edition, Oxford: Printed by John Lichfield and James Short, for Henry Cripps, 1624, OCLC 54573970:

     , New York 2001, p.75:

     Seneca […] accounts it a filthy lightness in men, every day to lay new foundations of their life, but who doth otherwise?

• nightless, slightens