Falling in sin is the same as committing sin. It is when you are tempted to sin and consciously agree to these temptations in your mind, allowing these desires to manifest as sinful thoughts, words, and deeds. However, your overall decision to serve God and live in victory has not changed. Even though you have made a mistake you still want to live free from sin. A fall is regarded as a “one-off” event, as opposed to living in sin with no intention of stopping.
The expression “the Fall” is not found in the Bible, but refers to the first time mankind (Adam and Eve) gave in to sin in the garden of Eden, thus corrupting their human nature, which has been passed down to all their descendants. (1 John 2:1; Revelation 2:4-5)