一面(yí miàn) - one side, one aspect, at the same time in Mandarin

一面 (yí miàn) is one side or one facet: of a coin, of an argument, of a person's character. 问题的一面 is one side of the issue and 温柔的一面 is a gentle side that not everyone sees.

Doubled up it becomes a construction rather than a noun. 一面 A, 一面 B means doing A and B at once: 一面走一面说. It is the slightly more written cousin of 一边...一边..., which is what you would say out loud to a friend.

The trap is the third use. 见过一面 does not mean seen one side of someone; it means met once, because 面 is also a measure word for meetings. 我们只见过一面 is a polite way of saying you barely know the person.