From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

A — 1: Μόνος (Strong'S #3441 — Adjective — monos — mon'-os )

denotes "single, alone, solitary,"  Matthew 4:4 , etc. See Only , Self.

B — 1: Μόνος (Strong'S #3441 — Adjective — monon — mon'-os )

the neuter of A, meaning "only, exclusively," e.g.,  Romans 4:23;  Acts 19:26 , is translated "alone" in the AV of  John 17:20; RV, "only." See Only.

B — 2: Καταμόνας (Strong'S #2651 — Adjective — kata monas — kat-am-on'-as )

signifies "apart, in private, alone,"  Mark 4:10;  Luke 9:18 . Some texts have the phrase as one word.

C — 1: Ἀφίημι (Strong'S #863 — Verb — aphiemi — af-ee'-ay-mee )

signifies "to send away, set free;" also "to let alone,"  Matthew 15:14;  Mark 14:6;  Luke 13:8;  John 11:48;  12:7 (RV, "suffer her"); in   Acts 5:38 some texts have easate from eao, "to permit." See Cry , Forgive , Forsake , Lay , Note (2), Leave , Let , OMIT, Put , No. 16, Note, Remit, Send, Suffer, Yield

 James 2:17 Mark 4:34

King James Dictionary [2]

ALO'NE, a. all and one.

1. Single solitary without the presence of another applied to a person or thing.

It is not good that man should be alone.  Genesis 2 .

This adjective follows its noun.

2. It is applied to two or more persons or things, when separate from others, in a place or condition by themselves without company.

And when they were alone, he expounded all things to his disciples.  Mark 4 .

3. Only.

Thou whose name alone is Jehovah.  Psalms 83 .

This sense at first appears to be adverbial, but really is not whose name single, solitary, without another, is Jehovah.

To let alone is to suffer to rest to forbear molesting or meddling with to suffer to remain in its present state. Alone, in this phrase, is an adjective, the word to which it refers being omitted let me alone let them alone let it alone that is, suffer it to be unmolested, or to remain as it is, or let it remain by itself.

ALO'NE, adv. Separately by itself.

Webster's Dictionary [3]

(1): (a.) Sole; only; exclusive.

(2): (adv.) Solely; simply; exclusively.

(3): (a.) Of or by itself; by themselves; without any thing more or any one else; without a sharer; only.

(4): (a.) Hence; Unique; rare; matchless.

(5): (a.) Quite by one's self; apart from, or exclusive of, others; single; solitary; - applied to a person or thing.