What is the meaning of Θ?

A plane angle.

a voiceless dental fricative.

[θ]-fricated release of a plosive (e.g. [tᶿ], sometimes implying an affricate [t͜θ]); [θ]-coloring; or a weak, fleeting or epenthetic [θ].

Lower-case theta (θῆτα), the 8th letter of the ancient Greek alphabet. It represented the voiceless aspirated dental or alveolar plosive /tʰ/ and later the voiceless dental fricative /θ/. It is preceded by η and followed by ι.

A letter of the Comox alphabet, written in the Latin script.

A letter of the Halkomelem alphabet.

The lower case letter theta (θήτα), the eighth letter of the modern Greek alphabet.

Used to represent -θar and -θe.

Source: wiktionary.org