Leviticus 13:4

If the bright spot be white in the skin of his flesh
The Targum of Jonathan is, white as chalk in the skin of his flesh; but other Jewish writers make the whiteness of the bright spot to be the greatest of all, like that of snow; (See Gill on Leviticus 13:2):

and in sight [be] not deeper than the skin, and the hair thereof
be not turned white;
though it be a bright spot, and be very white, yet these two marks not appearing, it cannot be judged a leprosy, at most it is only suspicious: wherefore

then the priest, shall shut up [him that hath] the plague seven
days;
in whom the bright spot is, and of whom there is a suspicion of the plague of leprosy, but it is not certain; and therefore, in order to take time, and get further knowledge, the person was to be shut up from all company and conversation for the space of seven days; by which time it might be supposed, as Ben Gersom observes, that the case and state of the leprosy (if it was one) would be altered; and Aben Ezra remarks, that most diseases change or alter on the seventh day.