[amen]
Amen means "so be it" or "it is certainly so."
[angel]
"Angel" literally means "messenger" or "envoy," and is usually used to refer to spiritual beings who normally are invisible to us, but can also appear as exceedingly strong creatures or as humans.
[Apollyon]
Apollyon is Greek for destroyer.
[apostle]
"Apostle" means a delegate, messenger, or one sent forth with orders. This term is applied in the New Testament in both a general sense connected with a ministry of establishing and strengthening church fellowships, as well as in a specific sense to "The 12 Apostles of the Lamb" (Revelation 21:14). The former category applies to a specific ministry that continues in the Church (Ephesians 4:11-13) and which includes many more than 12 people, while the latter refers to the apostles named in Matthew 10:2-4, except with Judas Iscariot replaced by Matthias (Acts 1:26).
[Armageddon]
See Har-magedon.