Boyd’s Bible Dictionary:

First letter of the Greek alphabet. Used with omega, the last letter, to express beginning and end (Isa. 41:4; 44:6; Rev. 1:8,11; 21:6; 22:13).

Concise Bible Dictionary:

The first letter of the Greek Alphabet, and which also signifies the numeral 1. A title or character of God and of Christ, which points to His eternity as “the beginning,” “the first,” the I AM (Rev. 1:8; Rev. 21:6; Rev. 22:13). “I am Alpha and Omega, the beginning and the end,” or “the first and the last”; which is similar to a passage in Isaiah 41:4: “I Jehovah, the first, and with the last; I am He.”

Strong’s Dictionary of Greek Words:

of Hebrew origin; the first letter of the alphabet; figuratively, only (from its use as a numeral) the first: --Alpha. Often used (usually ἄν, before a vowel) also in composition (as a contraction from 427) in the sense of privation; so, in many words, beginning with this letter; occasionally in the sense of union (as a contraction of 260).

Jackson’s Dictionary of Scripture Proper Names:

first letter of the Greek alphabet: beginning