Takbir (Arabic تكبير) is a verbal noun of the verb with the meaning kabbara say "Allahu akbar" (الله أكبر, DMG Allahu akbar, "God is greater (than anything else) is God (infinitely) large"). This formula is used very frequently in Islam.
Allah is the Arabic term for God. Akbar is the elative form of the adjective كبير kabir, "great, great, old, important", meaning "greater", "greatest" or "very large". The elative includes in its grammatical function of both the importance of the comparative (greater than) as well as on the superlative (biggest). The Arabic elative met but only in combination with the preposition min, the function of a comparison. For example: Allahu akbar min mala ikatihi ʾ ("Allah is greater than his angels"). Other respects the importance of the superlatives ("greatest").
In this meaning is already occupied takbir in Sura 17 verse 111: وكبره تكبيرا / wa kabbirhu takbīran / "and praise him everywhere." See also: Sura 74, verse 3, which is one of the oldest verse of the Quran : وربك فكبر / wa-fa-rabbaka kabbir / "And praise your Lord"
The expression is the beginning of the daily obligatory prayers (salat to speak), they call him: تكبيرة الإحرام / takbīratu ʾ l-ihram / "takbir consecration of state." In Islamic law, this takbir considered either as a religious duty and as part of prayer or Sunnah. During the prayer he repeated five times. Accordingly, the term also included in the voluntary prayers. The adhan call to prayer (begins) is also that term. According to tradition, Mohammed is to have a funeral at the takbir four or five times called. It is Unna prophets speak to takbir at various stations of the pilgrimage ceremonies at the sight of the Kaaba and at the end of a journey. According to some traditions, it is also prophets Unna, on seeing the new moon (hilal) call to start the fasting month of Ramadan "Allahu akbar."
Allahu akbar in the flag of Iraq.
In the flag of Iran as the lettering Allahu akbar ornament on red and green border is heavily abstracted.
The phrase "Allahu akbar" is included in the flag of Iraq and Iran and Afghanistan.
Source : http://de.wikipedia.org/wiki/Takb%C4%ABr