Shaykhul-Islam Ibn Taymiyyah (rahimahullah) said:
“Worship (Arabic : Ibadah) is obedience to Allah by carrying out what He commanded on the tongues of His messengers.”
He also said: “Ibadah is a collective term for everything which Allah loves and is pleased with from among the sayings and inward and outward actions.”
Ibn Katheer (rahimahullah) said:
“Ibadah in the (Arabic) language is humbleness (dhillah). It is sometimes said: ‘A levelled and smoothed road (mu’abbad) and an uneven road (ghayr 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 Allah’s worship).”
Ibn al-Qayyim (rahimahullah) said:
“Ibadah (is) gratefulness to Him, love of Him and awe of Him.”
Al-Qurtubi (rahimahullah) said: “The basis of ‘ibadah: humbling oneself (tadhallull) and submission (khudu’) and the duties of the Sharee’ah are called ‘Ibadat’ (pl. of ‘ibadah, i.e. acts of worship) because the (worshippers) stick to them and perform them with submissiveness and out of humility to Allah the Exalted.”
From these statements one will realise that ‘ibadah has a very wide meaning and is comprised of many matters. It will also become clear that many people have an incorrect and fragmented understanding of ‘ibadah. Most commonly they limit the meaning of ‘ibadah to prostrating, bowing, fasting, pilgrimage and other acts of worship. However, this is only one aspect from the many aspects of ‘ibadah.
In essence, the word ‘ibadah in the Arabic language and in the revelation has three meanings:
1) Humbleness, lowliness (dhillah) and submissiveness (khudu’)
2) Obedience (ta’ah) and compliance (inqiyad)
3) Devotion/sacrifice (tanassuk) and worship (ta’alluh)
When used in the Qur’an, either one of the above meanings is intended alone or two of them or all three together.
[ Source : The Book of Worship, Chapter 2 ]