"Science is the intellectual and practical activity encompassing the systematic study of the structure and behavior of the physical and natural world through observation and experiment." - Google Dictionary
"Science is the pursuit of knowledge and understanding of the natural and social world following a systematic methodology based on evidence." - Britain's Science Council
A nice discussion about this topic is given by Khalil A. Cassimally in Labcoat Life.
Carl Sagan has said:
He is the author of one of the best books about science ever written
Neil deGrasse Tyson has a nice quote about belief on science:
Let's not forget about Lesson 1673 though: