newel
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.
Joe Banister is 6' 3".
Nick Banister is 6' 1".
William Banister died in 1928.
William Banister was born in 1855.
The Crooked Banister was created in 1971.
Richard Banister died in 1626.
No, it is not a preposition. Banister is a noun. It is the railing of a staircase.
To replace a banister, you will need to remove the old banister by unscrewing it from the staircase and then install the new banister by attaching it securely to the staircase using screws or brackets. Make sure to measure carefully and follow any specific instructions for your particular banister design.