separate, apart, or isolated from others

I want to be alone.

to the exclusion of all others or all else

One cannot live by bread alone.

unique; unequaled; unexcelled

He is alone among his peers in devotion to duty.

solitarily; solely

She prefers to live alone.

only; exclusively.

without aid or help

The baby let go of the side of the crib and stood alone.

leave alone

to allow (someone) to be by himself or herself

Leave him alonehe wants to rest.

to refrain from annoying or interfering with

The youngsters wouldn't leave the dog alone, and he finally turned on them.

let alone

to refrain from annoying or interfering with.

not to mention

He was too tired to walk, let alone run.

let well enough alone

to be satisfied with the existing situation; refrain from attempting to change conditions

Marriages are often destroyed by relatives who will not let well enough alone.