Prayer means perceiving the true identity of the absolute, unmatched, utmost and glorious power, which is called God. The meaning of invocation is not something like performing ritual by muttering some holy verses, closing your eyes and praying for something good should happen to yourself and your family.
Invocation means performing any of your daily routines with honesty, sincerity and complete devotion to the work you perform without any expectations, personal gains and fruits of the work performed. This work sequentially offered to the supreme creator, sustainers, protector and the destroyer of all the evils as His services. When the mind, which both accepts and rejects, dissolves into the still, deep source, it is called the supreme worship.
At least trying to know the true identity of the God with sincerity, honesty and complete devotion is called invocation. Those who worship with the object of attaining a particular reward gain such reward which however is as destructible as a kingdom acquired in a dream. Those, however, who rightly act without hope of reward, attain salvation (nirvana) and are released from.