From BiblePortal Wikipedia

Vine's Expository Dictionary of OT Words [1]

Sôd ( סוֹד , Strong'S #5475), “secret or confidential plan(s); secret or confidential talk; secret; council; gathering; circle.” This noun occurs 21 times in biblical Hebrew.

Sôd means, first, “confidential talk”: “Hide me from the secret counsel of the wicked …” (Ps. 64:2). In Prov. 15:22 the word refers to plans which one makes on one’s own and before they are shared by others: “Without counsel [self-made] purposes are disappointed: but in the multitude of counselors they [succeed].” Sometimes the word signifies simply a talk about something that should be kept confidential: “Debate thy cause with thy neighbor himself; and discover not a secret to another” (Prov. 25:9).

Second, the word represents a group of intimates with whom one shares confidential matters: “O my soul, come not thou into their [Simeon’s and Levi’s] secret; unto their assembly, mine honor, be not thou united …” (Gen. 49:6—the first occurrence of the word). Jer. 6:11 speaks of the “assembly [informal but still sharing confidential matters] of young men together.” To “have sweet counsel” is to be in a group where everyone both shares and rejoices in what is being discussed and/or done (Ps. 55:14).

King James Dictionary [2]

SE'CRET, a. L. secretus. This is given as the participle of secerno, but is radically a different word. The radical sense of seg is to separate, as in L. seco, to cut off and not improbably this word is contracted into the Latin se, a prefix in segrego, separo, &c.

1. Properly, separate hence, hid concealed from the notice or knowledge of all persons except the individual or individuals concerned.

I have a secret errand to thee, O king.  Judges 3 .

2. Unseen private secluded being in retirement.

There secret in her sapphire cell,

He with the Nais wont to dwell. Fenton.

3. Removed from sight private unknown.

Abide in a secret place, and hide thyself.  1 Samuel 19 .

4. Keeping secrets faithful to secrets entrusted as secret Romans. Unusual. 5. Private affording privacy. 6. Occult not seen not apparent as the secret operations of physical causes. 7. Known to God only.

Webster's Dictionary [3]

(1): ( a.) Hidden; concealed; as, secret treasure; secret plans; a secret vow.

(2): ( a.) The parts which modesty and propriety require to be concealed; the genital organs.

(3): ( v. t.) To keep secret.

(4): ( a.) Faithful to a secret; not inclined to divulge or betray confidence; secretive.

(5): ( a.) Withdraw from general intercourse or notice; in retirement or secrecy; secluded.

(6): ( a.) Something studiously concealed; a thing kept from general knowledge; what is not revealed, or not to be revealed.

(7): ( a.) Separate; distinct.

(8): ( a.) A thing not discovered; what is unknown or unexplained; a mystery.

American Tract Society Bible Dictionary [4]

See Mystery

International Standard Bible Encyclopedia [5]

sē´kret  : In   Ezekiel 7:22 , English Versions of the Bible has "secret place" for (צפן , cāphan ), "hide," "treasure." A correct translation is, "They shall profane my cherished place" (Jerusalem), and there is no reference to the Holy of Holies. The other uses of "secret" in the Revised Version (British and American) are obvious, but Revised Version's corrections of the King James Version in   Judges 13:18;  1 Samuel 5:9;  Job 15:11 should be noted.