Anathema defined

Thayer’s Biblical Lexicon defines anathema as follows:

ἀνάθημα (anathema)
1. an offering resulting from a vow, which after being consecrated to a God was hung upon the walls or columns of the temple, or put in some other conspicuous place

2. a thing devoted to God without hope of being redeemed, and if an animal, to be slain; therefore a person or thing doomed to destruction

a) a curse
b) a man accursed, devoted to the direst of woes

Jimmy Akins teases out some of the word’s complexities in his article Anathema:

The Hebrew equivalent of anathema is kherem, which refers to a thing devoted to the Lord—a thing solemnly offered to God in a manner frequently involving its complete destruction. Kherem is often rendered in English by the terms “devoted thing,” “dedicated thing,” or thing placed “under the ban.” The Old Testament applies kherem to physical objects (Deut. 7:26, 13:17), livestock (1 Sam. 15:21), individual people (1 Kgs. 20:42), groups of people (Is. 34:5, 43:28), entire towns (Josh. 6:17), and lands or pieces of land (Lev. 27:21, Zec. 14:11, Mal. 4:6). Things to be placed under the ban by men were either destroyed (Lev. 27:28) or given to the priests (Num. 18:14, Ezek. 44:29). A land under the ban was a land that had been cursed (Zec. 14:11, Mal. 4:6). Paradoxically, something could be kherem either because it was holy or because it was unholy.

The Greek term anathema shares something of this paradox. It is derived from the roots ana– (on, upon, among, between) and tithemi (to place, put, set). Etymologically, the word suggests something placed among the holy things (i.e., in a temple)—a sense preserved in the variant term anathema (Luke 21:5). The more common anathema has the sense of a curse and is applied in the New Testament to a curse by which individuals bind themselves (Acts 23:14).

But to truly understand it we must go much further back – to William Smith’s A Dictionary of Greek and Roman Antiquities:

DONA′RIA (ἀναθήματα or ἀνακείμενα), are names by which the ancients designated presents made to the gods, either by individuals or communities. Sometimes they are also called dona or δῶρα. The belief that the gods were pleased with costly presents was as natural to the ancients as the belief that they could be influenced in their conduct towards men by the offering of sacrifices; and, indeed, both sprang from the same feeling. Presents were mostly given as tokens of gratitude for some favour which a god had bestowed on man; but in many cases they were intended to induce the deity to grant some special favour.

At the time when the fine arts flourished in Greece the anathemata were generally works of art of exquisite workmanship, such as high tripods bearing vases, craters, cups, candelabras, pictures, statues, and various other things. The materials of which they were made differed according to circumstances; some were of bronze, others of silver or gold (Athen. VI p231, &c.), and their number is to us almost inconceivable (Demosth.Olynth. III. p35). The treasures of the temples of Delphi and Olympia, in particular, surpass all conception. Even Pausanias, at a period when numberless works of art must have perished in the various ravages and plunders to which Greece had been exposed, saw and described an astonishing number of anathemata.

Individuals who had escaped from some danger were no less anxious to show their gratitude to the gods by anathemata than communities. In all cases in which a cure was effected presents were made to the temple, and little tablets (tabulae votivae) were suspended on its walls, containing an account of the danger from which the patient had escaped, and of the manner in which he had been restored to health. Some tablets of this kind, with their inscriptions, are still extant (Wolf, l.c., p242, &c.). From some relics of ancient art we must infer, that in some cases, when a particular part of the body was attacked by disease, the person, after his recovery, dedicated an imitation of that part in gold or silver to the god to whom he owed his recovery. Persons who had escaped from shipwreck usually dedicated to Neptune the dress which they wore at the time of their danger (Hor. Carm. I.5.13;Virg. Aen. XII.768); but if they had escaped naked, they dedicated some locks of their hair (Lucian, de Merc. Cond. c1 vol. I p652, ed. Reiz.). Shipwrecked persons also suspended votive tablets in the temple of Neptune, on which their accident was described or painted. Individuals who gave up the profession or occupation by which they had gained their livelihood, frequently dedicated in a temple the instruments which they had used, as a grateful acknowledgment of the favour of the gods. The soldier thus dedicated his arms, the fishermen his net, the shepherd his flute, the poet his lyre, cithara, or harp, &c.

It would be impossible to attempt to enumerate all the occasions on which individuals, as well as communities, showed their gratefulness towards the gods by anathemata. Descriptions of the most remarkable presents in the various temples of Greece may be read in the works of Herodotus, Strabo, Pausanias, Athenaeus, and others.