a finial
The common term is "banister".
The post actually is the banister. The fat bit that you slide your hand on is called a handrail or a balustrade.
A banister is a railing up the side of stairs.
No, it is not a preposition. Banister is a noun. It is the railing of a staircase.
The Crooked Banister was created in 1971.
William Banister died in 1928.
William Banister was born in 1855.
Richard Banister died in 1626.
Joe Banister is 6' 3".
Nick Banister is 6' 1".
You can repair a banister by first removing or stripping the nails, sanding the area and reapplying a fresh banister.