The five books of the Torah—like the 54 parshiyyot—are by tradition each named after their first significant word or phrase.  In the case of the fourth book, the name is taken from half of a semikhut (construct) phrase: “בְּמִדְבַּר סִינַי - in the Sinai Desert” (bemidbar Sinai).  The custom has developed to use just the first of the two words: bemidbar, meaning just: “in the Desert.”  That leaves us with a particularly evocative title, one that casts us out into a vast unknown, and vaguely suggests impending danger.