Definition of mean solar day in English:

mean solar day

noun

Astronomy
  • The time between successive passages of the mean sun across the meridian.

    • ‘It no longer made sense to define the second as 1 / 86,400th of a mean solar day.’
    • ‘The second was defined to be 1 / 86,400 of the mean solar day, the average rotational period of the earth with respect to the Sun.’