The following is an excerpt from a lecture given by Shrila Prabhupada in 1968.
"In the gross material concept of life we are under the impression that "I am this body." Therefore we are concerned with the senses. If our senses are gratified, we think we are now satisfied. So this is the gross type of existence, I mean to say, existence of ignorance. Illusion. Maya. When one is under the thought that "I am this," this is illusion. Illusion means you accept something, something is presented as reality, and you accept it.
Just like the example is given, water in the desert. Mirage. There is no water, but a..., an animal is hankering, is running after water in the desert. That is practical, that due to sunshine there is a reflection, it appears in the desert. Sometimes you might have seen -- not here; in India we have seen several times -- that exactly as vast water, and it is reflecting, the reflection.
That is called mirage. There is not a drop of water, but the animal, when he is thirsty, he..., it thinks that "There is water." He jumps into the desert and the water is going ahead, going ahead, and he is running after it and then dies. So this illusion, that "I am this body." So we are after this sense gratification. Body means the senses. So that is mirage, illusion, just like the animal is running after water in the desert."
(Bhagavad-gita As It Is Introduction – November 23, 1968 , Los Angeles)