I sometimes hear these nouns used interchangeably.

Problem:
These nouns are not all synonyms.

Explanation:
A “baluster” is a moulded shaft that most often supports the handrail of a staircase but also can be used to support the coping of a parapet, which is a wall-like barrier often at the edge of a roof.

The noun “baluster” comes from the Italian word — balaustra — for a pomegranate flower because the traditional shape of a baluster resembles the half-open pomegranate flower.

Balusters often are made of stone or wood and are sometimes made of metal.

A “balustrade” is a sequence of balusters that support a handrail.

A “banister” — also spelled with two “n”s — is another name for a staircase baluster, but many architects and interior designers prefer to reserve the word “banister” for a narrower, more-modern support.

Now here is where it becomes more confusing.

Many Americans — I cannot speak for other countries — also use the word “banister” to refer to the handrail of a staircase.

Solution:

• Use “baluster” to refer to a pomegranate-flower-shaped support of a staircase handrail.
• Use “balustrade” to refer to a sequence of balusters.
• Use “banister” to refer to a non-pomegranate-flower support of a staircase handrail.
• Be aware that some use “banister” also to refer to the staircase handrail itself.