Both the singular and plural of means end in –s. ‘Means’ means method, process, way etc.

The railway is perhaps the most popular means of communication.

Few means of transport are available on the island.

By all means

This expression is used to give permission to do something. It expresses ideas such as ‘of course’ and ‘it is all right to…’

‘Can I borrow your car?’ ‘By all means.’

‘By all means buy a new camera, but don’t spend more than 300 dollars.’ (= It is all right to buy a new camera, but …)

By all possible means does not mean the same as by all means. It has a similar meaning to ‘at any cost’.

We must prevent this by all possible means.  (= We must prevent this at any cost.)

By no means

By no means has a similar meaning to ‘definitely not’.

‘Is this all you want?’ ‘By no means.’ (= No, I want more.)

Columbus was by no means the first person to discover America. People had been living there long before he ‘discovered’ it.

By means of (= through)

I opened the tin by means of a tin opener. (= I opened the tin with the help of a tin opener.)

By some means or other = somehow or other.

A man of means = a rich man

Live within one’s means = not spend more than one’s income.