Boyd’s Bible Dictionary:
Place of departed spirits
Greek equivalent of Hebrew “sheol,” unseen world. Hell in A. V.; Hades in R. V. (Matt. 11:23; 16:18; Acts 2:31; Rev. 1:18).
Concise Bible Dictionary:
See HELL.
From Anstey’s Doctrinal Definitions:
“Sheol” and “Hades” refer to the same thing; Sheol is Old Testament Hebrew and Hades is New Testament Greek. (See the J. N. Darby Translation footnote on Psalm 6:5.) Sheol/Hades is the world of unseen disembodied souls and spirits into which deceased believers and unbelievers pass, without specifying what condition they are in. (See the J. N. Darby Translation footnote on Matthew 11:23.) The gospel has brought to light “life and immortality” (2 Tim. 1:10), and as a result, we now know that there are two opposing conditions in Sheol/Hades: "torments" for the lost (Luke 16:24-25) and bliss ("paradise" –“the garden of delights”) for believers (Luke 23:43).
What can be confusing is that Hades is erroneously translated as "hell" eleven times in the KJV (Matt. 11:23; 16:18; Luke 10:15; 16:23; Acts 2:27, 31; 1 Cor. 15:55 – margin; Rev. 1:18; 6:8; 20:13, 14). Each one of these references should be translated "Hades." Consulting a critical translation, such as the J. N. Darby Translation, clears this up immediately.
A query was submitted to the editor of Help and Food magazine concerning Hades: "Ques. What is Hades? Ans. Unquestionably it is the whole unseen world, including both saved and lost. See Luke 16:23; Revelation 20:13-14, for the lost, and Acts 2:27, 31 for our blessed Lord. Hades answers to Sheol in the Old Testament" (Help and Food, vol. 14, p. 140). We would add to these references 1 Corinthians 15:55, which indicates that believers who have died are in Hades. The KJV renders it “grave” in this verse, but the word in the Greek is “Hades” and should be translated as such (J. Green’s Interlinear; Wigram’s Englishman’s Greek Concordance; Strong’s Concordance, etc.).
Sheol/Hades is a temporary state. All in this intermediate or separate state, into which death brings a person, will be resurrected—both believers and unbelievers—but at different times. (See Resurrection.) All who have died in faith are "unclothed" in Hades (2 Cor. 5:4). Their souls and spirits are "with Christ" (Phil. 1:23) in “paradise” (Luke 23:43). This is in heaven because Scripture says that that is where Christ is (Luke 24:51; Acts 1:9-10; 3:21; 7:55; Phil. 3:20; Heb. 4:14). Paul confirms this, stating that when he was disembodied, he was in “paradise” in “the third heaven” (2 Cor. 12:2-4). The Lord also taught that the disembodied souls and spirits of children who have died under the age of responsibility go straight to "heaven" (Matthew 18:10 – "their angels" is a reference to their disembodied spirits; see Acts 12:15). The bodies of these disembodied persons, however, currently lay in the grave.
The souls and spirits of all who have died without faith are also in Hades, but they are in “torments” (Luke 16:23). They will remain in that state until the end of time, at which time they will be resurrected and judged for their sins at the “great white throne” (Isa. 24:22; Rev. 20:11-15). They will then be cast into “the lake of fire,” which is Hell. Hence, contrary to what is commonly thought, those who have died in their sins are not in Hell yet, and when they are put there, they will not be dead. They will be raised to life again and cast there alive! (Rev. 20:5; 13)