It can be the outright effort to deceive by telling an untruth. It can take the form of slightly twisting the truth to deceive. Any time a man speaks one thing, and either knows or thinks something else, he is speaking falsehood. This the believer should avoid. In the present case Paul condemns falsehood because of what it does to one's relation to others in the same union with Christ.
Believers are members of one body.
Lying is not just condemned for ethical reasons, but because all mutual duties of Christians are based on their union with Christ. This unity and community with other Christians is damaged when falsehood enters the relationship. That which damages relations with other Christians damages our relationship with Christ. Growing into Him who encompasses all truth brings about purity of mind from which honest words and thoughts flow. He who walks uprightly, and works righteousness, and speaks the truth in his heart.