Wilson's Dictionary of Bible Types 
The number which indicates something new.
1 Samuel 17:12 (b) David was the eighth son and he began a new dynasty in Israel.
Ecclesiastes 11:2 (c) Seven, being the number of perfection, this is an invitation to go further than the full amount required. The fulfillment of the expression "go the second mile."
John 20:26 (b) This is a new revelation to Thomas and a new confession from him.
1 Peter 3:20 (c) The eight souls in the ark represented a new manifestation of GOD's grace, a new exhibition of His terrible judgment, and a new message to the world. The number "eight" always represents something new. This is true both in Scripture and in nature. The chapters numbered "eight" contain a new revelation not previously found in the Scriptures. The eighth note on the piano begins a new octave, The eighth day of the week begins a new week. There are just seven colors in the rainbow; the eighth color begins a new spectrum.
ge8 - is the beginning of a new generation of people on the earth proceeding from Noah and his family.
ex8 - is the performing of a miracle which had heretofore been unknown in the history of man.
mt8 - is the first instance of the miraculous cleansing of the leper by the word and the touch of another.
lu8 - is a new revelation of relationship between Christ and those who hear His Word; a new experience of seven demons cast out of one person.
ac8 - is a new experience of Phillip in personal work, the leading of the Spirit, and the conversion of an Ethiopian.
ro8 - brings a new revelation of the leading of the Spirit.
heb8 - shows a new revelation of GOD's covenant.
re8 - gives a new vision of the ministry of angels.
All the other chapters number eight in the Bible express something new as these do.
Webster's Dictionary 
(1): ( n.) An island in a river; an ait.
(2): ( a.) Seven and one; as, eight years.
(3): ( n.) The number greater by a unit than seven; eight units or objects.
(4): ( n.) A symbol representing eight units, as 8 or viii.