Shaikhul-Islaam Ibn Taymiyyah (rahimahullaah) said: "Ibaadah is obedience to Allaah by carrying out what He has commanded on the tongues of His messengers."
He also said: "Ibaadah is a collective term for everything which Allaah loves and is pleased with from among the sayings and inward and outward actions."
Ibn al-Qayyim said: Ibaadah - (is) Gratefulness to Him, Love of Him and Awe of Him
Ibn Katheer said:"Ibaadah in the language is humbleness (dhillah). It is sometimes said 'a levelled and smoothed road (mu'abbad) and an uneven road (ghair mu'abbad)' meaning (one has been) humbled, lowered, debased. Its usage in the Revelation: an expression of what brings together the perfection of Love, Fear and Hope (in Allaah's worship)."
Al-Qurtubee said: "The basis of Ibaadah: humbling oneself (tadhallull) and submission (khudoo') and the duties of the Sharee'ah are called 'Ibaadaat' (pl. of Ibaadah i.e. acts of worship) because the (worshippers) stick to them and perform them with submissiveness and out of humility to Allaah the Exalted."
From these statements you will realise that Ibaadah has a very wide meaning and is comprised of many matters. It will also become clear to you that many people have an incorrect and fragmented understanding of Ibaadah. Most commonly they limit the meaning of Ibaadah to prostrating, bowing, Fasting, Hajj and other acts of worship. However, this is only one aspect from the many aspects of Ibaadah.
In essence, the word Ibaadah in the language and in the Revelation has three meanings:
 Humbleness, Lowliness (Dhillah) and Submissiveness (Khudoo')
 Obedience (Taa'ah) and Compliance (Inqiyaad)
 Devotion, Sacrifice (Tanassuk) and Worship (Ta'alluh)