KEY vs. INDEX

[David T. Ashley]

In reading the syntax of the CREATE TABLE statement, I'm a little confused
by the two keywords KEY and INDEX.

What does one use when one wants MySQL to arrange the column so that it can
find a given record "WHERE column=whatever" in approximately O(log N) time?

This is a "key", right?  MySQL uses some kind of a BTREE arrangement, right?

Thanks.