It is a denial of the full story, by hiding something or giving a distorted view through a mixture of what is true with what is false or incomplete, in an effort to deceive or impress, to justify or excuse behaviour. Half-truth is not the truth but misinformation. Full and complete truth is important to God, whereas Satan operates in the realm of half-truths. Satan is not too concerned when people have just enough knowledge or truth about God to appear Biblical and consider they will go to heaven when they die because of the concept that good people go to heaven. Such people don't understand there is no salvation without repentance, but are deceived by the cheap gospel that propagates God's love and grace without us living by His standards ; it is denying the power of the gospel to save and transform (2 Tim 3:1-5). The Bible says, as we know the real and complete truth we can be set free (Jn 8:31,32). It is of utmost importance to know and live by the truth of God's Word, so not to be deceived by Satan's lies or following the opinions and nice ideals of man.
Some of the half-truth examples in Scripture are when Satan:
1/. told Eve she'd be like God, knowing good and evil (Gen 3:4,5). While Adam and Eve's eyes were open to see good and evil (this was true), they did not become like God but less like Him because of disobedience and sin. As a result of this deception all humanity continues to suffer.
2/. inspired Ananias and Sapphira to give a false impression that they had given much more than in reality they had. Note: they were under no compulsion to give any at all. God severely judged them for their half-truth giving and they both died on the spot (Act 5:1-10).
By not telling the whole truth, problems become more complex rather than easier (Gen 12:10-20, 20:11). As Christians we should be completely open and honest with others, with truthfulness being our standard. However, in many situations not everything known needs to be said, as this only leads to the escalation of gossip. Wisdom is needed to know what and when to say something as our speech is a major cause of sin (Jas 3:5). “Help me, Lord, to keep my mouth shut and my lips sealed” (Ps 141:3).