Fill in the blanks with the appropriate word, phrase, or symbol(s) from the following list.
$$\begin{array}{lllllr}\text { odd } & \text { index } & \text { principal } & \text { cube } & \text { rational } & \text { radical } \\\text { square } & \text { radicand } & \text { even } & \text { irrational } & \text { negative }.\end{array}$$
The _________ square root of $a$, written $\sqrt{a}$, is the positive number $b$ such that $b^{2}=a$.