From BiblePortal Wikipedia

Vine's Expository Dictionary of OT Words [1]

A. Nouns.

Rêa‛ ( רֵיעַ , Strong'S #7453), “friend; companion.” This noun is also represented in Akkadian, Ugaritic, and Aramaic. Rêa‛ appears 187 times in the Hebrew Old Testament, and it has an extensive range of meaning.The basic meaning of rêa‛ is in the narrow usage of the word. A rêa‛ is a “personal friend” with whom one shares confidences and to whom one feels very close: “And the Lord spake unto Moses face to face, as a man speaketh unto his friend” (Exod. 33:11). The closeness of relationship is best expressed by those texts where the rêa‛ is like a brother or son, a part of the family: “For my brethren and companions’ sakes …” (Ps. 122:8, cf. Deut. 13:6). For this reason, when Zimri became king over Israel he killed not only all relatives of Baasha, but also his “friends” (1 Kings 16:11). In this sense, the word is a synonym of ‘ah (“brother”) and of qarob (“kin”): “… Go in and out from gate to gate throughout the camp, and slay every man his brother, and every man his companion and every man his neighbor” (Exod. 32:27).

Similar to the above is the sense of “marriage partner”: “His mouth is most sweet: yea, he is altogether lovely. This is my beloved, and this is my friend O daughters of Jerusalem” (Song of Sol. 5:16). However, rêa‛ may also signify “illegitimate partners”: “… If a man put away his wife, and she go from him, and become another man’s, shall he return unto her again? shall not that land be greatly polluted? but thou has played the harlot with many lovers ( rêa‛ ); yet return again to me, saith the Lord” (Jer. 3:1). The prophet Hosea was commanded to take back his wife from her “friend” (lover), as she had played the adulteress long enough.

The wider usage of rêa‛ resembles the English word neighbor the person with whom one associates regularly or casually without establishing close relations. One may borrow from his “neighbor” (Exod. 22:14), but not bear false witness (Exod. 20:16) nor covet his neighbor’s possessions (Exod. 20:17-18). The laws regulate how one must not take advantage of one’s “neighbors.” The second greatest commandment, which Jesus reiterated—“Love thy neighbor as thyself” (Lev. 19:18)—receives reinforcement in the laws of the Pentateuch. The prophets charged Israel with breaking the commandment: They oppressed each other (Isa. 3:5) and desired their neighbors’ wives (Jer. 5:8); they committed adultery with these women (Ezek. 18:6); they did not pay wages to the worker (Jer. 22:13); and they improperly took advantage of their “neighbors” (Ezek. 22:12). According to Proverbs, not loving one’s neighbor is a sign of foolishness (Prov. 11:12).

The wider meaning comes to expression in the proverb of the rich man and his “friends”: “Wealth maketh many friends; but the poor is separated from his neighbor” (Prov. 19:4). Here the “friend” is a person whose association is not long-lasting, whose friendship is superficial.

The Septuagint gives the following translations: plesion —(“near; close by”), philos —(“friend”). The KJV gives these senses: “neighbor; friend; fellow; companion.” Re’eh —also means “friend.” This noun appears in 1 Kings 4:5: “… Zabud the son of Nathan was principal officer, and the king’s friend .…”

Re’ah —refers to a “female friend.” See Judg. 11:37 for this usage: “And she said unto her father … let me alone two months, that I may go up and down upon the mountains, and bewail my virginity, I and my fellows —" (cf. Judg. 11:38; Ps. 45:14). The noun re’ut means “beloved companion; bride.” Ra’yah —occurs many times in the Song of Solomon: 1:9, 15; 2:2, 10, 13; 4:1, 7; 5:2; 6:4. Re’ut —refers to a “fellow woman.” This word is usually translated idiomatically in a reciprocal phrase of “one another,” as in Zech. 11:9: “Then said I, I will not feed you: that that dieth, let it die; and that that is to be cut off, let it be cut off; and let the rest eat every one the flesh of another. "

B. Verb.

Râ‛âh ( רָעָה , Strong'S #7462), “to associate with.” This word appears in Prov. 22:24: “Make no friendship with an angry man; and with a furious man thou shalt not go.…”

Vine's Expository Dictionary of NT Words [2]

1: Συνέκδημος (Strong'S #4898 — Noun Masculine — sunekdemos — soon-ek'-day-mos )

"a fellow-traveler" (sun, "with," ek, "from," demos, "people;" i.e., "away from one's people"), is used in  Acts 19:29 , of Paul's companions in travel; in  2—Corinthians 8:19 , "travel with;" a closer rendering would be "(as) our fellow-traveler." See Travel.

2: Κοινωνός (Strong'S #2844 — Noun Masculine — koinonos — koy-no-nos' )

is rendered "companions" in the AV of  Hebrews 10:33 (RV "partakers"). So sunkoinonos in   Revelation 1:9 , AV, "companion;" RV, "partaker with you." See B, above, Partaker , Partner. Cp. Communicate

3: Συνεργός (Strong'S #4904 — Adjective — sunergos — soon-er-gos' )

"a felloworker" (sun, "with," ergon, "work"), is translated in  Philippians 2:25 "companion in labor," AV (RV, "fellow-worker"). See Helper , Laborer , Worker.

King James Dictionary [3]

Companion n.

1. One who keeps company with another one with whom a person frequently associates, and converses. It differs from friend, says Johnson, as acquaintance from confidence. The word does not necessarily imply friendship but a companion is often or generally a friend.

A companion of fools shall be destroyed.  Proverbs 13 .

2. One who accompanies another as two persons meeting casually and traveling together are called companions. So soldiers are called companions in arms. 3. A partner an associate.

Epaphroditus, my brother, and companion in labor, and fellow soldier.  Philippians 2 .

4. A fellow a mate. 5. A sort of wooden porch placed over the entrance or stair case of the cabin in merchant ships. Hence the ladder by which officers ascend to and descend from the quarter deck is called the companion ladder.

Webster's Dictionary [4]

(1): (n.) A knight of the lowest rank in certain orders; as, a companion of the Bath.

(2): (n.) One who accompanies or is in company with another for a longer or shorter period, either from choice or casually; one who is much in the company of, or is associated with, another or others; an associate; a comrade; a consort; a partner.

(3): (n.) A fellow; - in contempt.

(4): (n.) A skylight on an upper deck with frames and sashes of various shapes, to admit light to a cabin or lower deck.

(5): (n.) A wooden hood or penthouse covering the companion way; a companion hatch.

(6): (v. t.) To be a companion to; to attend on; to accompany.

(7): (v. t.) To qualify as a companion; to make equal.