最中(さいちゅう)にIn the middle of

While 中(ちゅう)に highlights that something is in the process of happening (when used in relation to events/actions), 最中(さいちゅう)に strengthens that meaning further, to show that something is 'right in the middle of happening'. As the kanji 最(さい) means 'utmost', we can see that the literal meaning is 'utmost middle'. This can be translated in a similar way to phrases like 'at the height of (A)', or 'in the midst of (A)', in English.

When using 最中(さいちゅう)に, a (B) part of the sentence will usually highlight something else that happened during (A), somehow interrupting or disturbing it.

To use 最中(さいちゅう)に, attach it to the end of the ている form of a verb, or a noun followed by の.

Caution

最中(さいちゅう) may also occasionally be used with adjectives, but this is uncommon. In these cases, the meaning is to be at the 'pinnacle' of whatever the adjective is describing (usually a feeling/emotion).