injurious speech that insults and tears down its target: blasphemy when aimed at God, slander when aimed at people.
One Greek word covers a whole spread of abusive speech, from cursing God to running down a neighbor. What holds it together is not the target but the intent: speech meant to insult, defame, and drag something down. English has no single word for that, so translations split it in two. When the target is God or what is sacred, they render it "blasphemy"; when the target is another person, they render it "slander," "reviling," or "evil speaking."
The God-directed use is the sharper one. It is the charge brought against Jesus for claiming what belongs to God alone, and it is the roaring, God-defying speech of the beast in Revelation. The most solemn instance is the warning about speaking against the Holy Spirit.
The people-directed use sits in the New Testament lists of things a changed heart leaves behind, alongside anger, malice, and filthy talk. Here it is close in force to plain verbal abuse: tearing someone down with words, whether or not the charge is even true.
Because the two shades share a single word, the New Testament can move between them easily, and a reader who only hears "blasphemy" as a religious term can miss that the same word is also the ordinary word for slander. Two close relatives narrow the people-directed side: λοιδορία is abusive railing to someone's face, and καταλαλιά is running someone down behind their back.