There are four Greek words which are thus translated; and it is most important that they should be, in each occurrence, carefully distinguished.
They are as follows:—

 1.  Kosmos = the world as created, ordered, and arranged. Hence it is used in the LXX (Septuagint) for the Hebrew word rendered "ornament". See Exodus 33:5, 6. Isaiah 49:18. Jeremiah 4:30. Ezekiel 7:20, etc. It denotes the opposite of what man has called "chaos", which God never created. See notes on Isaiah 45:18 and Genesis 1:2: for the Hebrew bara' means not only to create, but that what was created was beautiful. The root, meaning to carve, plane, polish, implies both order and beauty. Compare Appendix 146.

 2.  aion = an age, or age-time, the duration of which is indefinite, and may be limited or extended as the context of each occurrence may demand.
  The root meaning of aion is expressed by the Hebrew 'olam (see Appendix 151. I. A and II. A) which denotes indefinite, unknown or concealed duration: just as we speak of "the patriarchal age", or "the golden age", etc. Hence, it has come to denote any given period of time, characterized by a special form of Divine administration or dispensation.
  In the plural we have the Hebrew 'olamim and Greek 'aiones used of ages, or of a succession of age-times, and of an abiding from age to age. From this comes the adjective aionios (Appendix 151. II. B), used of an unrestricted duration, as distinct from a particular or limited age-time. These age-times must be distinct or they could not be added to, or multiplied, as in the expression aions of aions.
  These ages or age-times were all prepared and arranged by God (see Hebrews 1:2; 11:3); and there is a constant distinction in the New Testament between "this age", and

the "coming age" (see Matthew 12:32. Hebrews 1:2. Ephesians 1:21).
  "This age" is characterized by such passages as Matthew 13:24-30, 36-43. Mark 4:19; 10:30. Romans 12:2. 1Corinthians 2:8. 2Corinthians 4:4. Galatians 1:4. Ephesians 2:2. (translated "course"). 2Timothy 4:10. Titus 2:12.
  The "coming age" is characterized in such passages as Matthew 13:39, 40, 49; 24:3; 28:20. Mark 10:30. Luke 18:30; 20:35. 1Corinthians 15:23. Titus 2:13.
  The conjunction of these ages is spoken of as the sunteleia, marking the end of one age and the beginning of another.
  Other indefinite durations are mentioned, but they always refer to some unknown and prolonged continuance, the end of which cannot be seen; such as the end of life (Exodus 21:6). Hence the Hebrew Priesthood was so characterized because its end could not be foreseen (see Exodus 40:15. 1Samuel 1:22. Hebrews 7:12). It is used in the same way in other connections (see Matthew 21:19. John 8:35). For further information see Appendix 151. II. A.

 3.  oikoumene = the world as inhabited. It is from the verb oikeo = to dwell. It is used of the habitable world, as distinct from the kosmos (Number 1 above, which = the world as created). Hence, it is used in a more limited and special sense of the Roman Empire, which was then predominant. See Luke 2:1; 4:5; 21:26. It is sometimes put by the Figure of Speech Metonymy (of the Adjunct), Appendix 6, for the inhabitants (Acts 17:6, 31. Hebrews 2:5, etc.).

 4.  ge = land, as distinct from water; or earth as distinct from heaven; or region or territory, used of one special land, or country, as distinct from other countries, in which peoples dwell, each on its own soil.