The thoughts on this post began with a pastor asking the question, "Would you mind telling me how I as a pastor can know when a member of my flock is guilty of drunkenness?" The Bible-believing Christian must answer this the same way regardless of his or her position on drinking in moderation or abstention. Go to the Bible and find out what it says about drunkenness. We won't find a fine dividing line that we can determine with a breathalyzer. That is a secular and legal approach. In our state we legally define that point with blood alcohol level -- 0.08 blood alcohol concentration (at least while driving).1 BUT the question now before us is how the Bible, our rule of faith and practice, defines it. There we will find guidance. Inspired, inerrant guidance at that!
Please consider the following points from Scripture.
There is a point at which one passes from "drinking wine" to drunkenness; that is, these are two different states. For examples, Genesis 9:21 "And he drank of the wine, and was drunken..." Deuteronomy 29:19 "And it come to pass, when he heareth the words of this curse, that he bless himself in his heart, saying, I shall have peace, though I walk in the imagination of mine heart, to add drunkenness to thirst:"
The Bible speaks of "drunken" but also of an exceeding state of that: 1 Samuel 25:36 "And Abigail came to Nabal; and, behold, he held a feast in his house, like the feast of a king; and Nabal's heart was merry within him, for he was very drunken" The "very drunken" state is most obvious, but drunken might not be quite as readily observable.
Certain actions, states or physical signs are often associated with drunkenness in the Bible, such as staggering, shaking and vomiting. Psalm 107:27 They reel to and fro, and stagger like a drunken man, and are at their wit's end. (Cf. Jer. 23:9, Isa. 19:14; Prov. 23:29). [We understand other things can also cause these signs, such as sickness.]
Actions or states unrelated to alcohol sometimes are mistaken for drunkenness. 1 Samuel 1:13 "Now Hannah, she spake in her heart; only her lips moved, but her voice was not heard: therefore Eli thought she had been drunken." (Cf. Acts 2:15)
Drunkenness is associated with loss of control of one's senses or actions. Sometimes "loss of control" can be in a good way -- being controlled by the Spirit. Ephesians 5:18 "And be not drunk with wine, wherein is excess; but be filled with the Spirit;" There are also other figures in the Bible that speak of being "drunk" on something other than wine/strong drink that seem to generally share the meaning of not being in control or possession of one's faculties. (Cf. Job 12:24-25; Lam. 4:21; Rev. 17:2)
Drunkenness is caused by an excess of wine. Compare Eph. 5:18 and I Peter 4:3. For the time past of our life may suffice us to have wrought the will of the Gentiles, when we walked in lasciviousness, lusts, excess of wine, revellings, banquetings, and abominable idolatries:
This may be simplistic. Perhaps it does not draw as exact a line as you may want. But looking at these verses are a starting place for a BIBLICAL definition of drunkenness.
1. I'm not sure if it is proper to say this defines drunkenness legally so much as to say it is the point where driving becomes illegal. For example, I don't think BAC has anything to do with "public intoxication" in Texas, which appears to be at a law officer's discretion.