The word "souls" (psuchás, plural of psuché) also requires explanation, as the Greek word is far too complex in meaning to define facilely as a person's immortal essence, as most Catholics and Protestants are wont to do. Its basic meaning is "breath," and is thus equivalent to the Hebrew nephesh and Latin anima (as in English "animal" and "animate"). One of its uses is as the New Testament version of what Genesis 2:7 calls "the breath of life," that is, the vital force that makes a body live: "And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being [nephesh]." Luke 12:20 and Acts 20:10 use psuché in this manner.
From this basic meaning derives its extensions: as "life" (see Matthew 6:25; John 10:11; Philippians 2:30; Revelation 12:11) and "living being" (see I Corinthians 15:45; Revelation 16:3). In addition, psuché can refer to the seat of emotion, will, and desire, whereas we would use the terms "heart," "mind," "personality," or "being" today (see Luke 1:46; Acts 14:2, 22; Hebrews 6:19; II Peter 2:14). In a similar sense, it can also identify man's moral and spiritual life (see Hebrews 13:17; I Peter 1:22; 2:11, 25; 4:19; III John 2).
Some try to read immortality into certain biblical uses of psuché (for instance, Acts 2:27, 31; II Corinthians 1:23; Revelation 20:4), but the Bible does not support such an interpretation. In fact, in one of these, Matthew 10:28, Jesus confirms that souls can indeed be destroyed (also supported by the Old Testament in Job 33:22; Ezekiel 18:4, 20)! One must consult extrabiblical sources (such as Plato, Xenophon, Herodotus, and other Greek writers) to find usages of psuché that define "the soul as an essence which differs from the body and is not dissolved by death" (Thayer's Lexicon).
How then is this word used in Revelation 6:9? We must remember that John is viewing a vision (Revelation 1:10), a symbolic representation for mortal eyes and minds of future events, not reality. One cannot see a person's actual soul, that is, his being, his life, so what John saw were representations of those who had been martyred. He probably literally saw bodies (Greek soma) under the altar but chose to identify them as psuchás, "lives" or "persons," because, as the next verses show, the vision depicts them speaking and receiving clothing, things a person can do only while alive.
The important point to remember is that John specifically identifies them as having been "slain"—they are dead—and the Bible elsewhere shows that "the dead know nothing" (Ecclesiastes 9:5) and cannot work, plan, learn, or pursue any activity in the grave (verse 10). Thus, John, a Hebrew, is using psuché in the same sense as Old Testament writers sometimes use nephesh, as "dead body," a being that once had life (see Leviticus 21:11; Numbers 6:6; 9:6-7, 10; 19:11, 13; Haggai 2:13).