adj, adv with one member of one group corresponding to one of another 一对一（的）; 一比一（的）
*a ,one-to-one `ratio between teachers and pupils 师生比例一比一
* teaching one-to-one 一对一地教授.
between only two people
tuition on a one-to-one basis
matching each other exactly
a one-to-one correspondence between letters and sounds
I need to discuss it with him one-to-one.