最中(さいちゅう)にIn the middle of

Both can be translated as 'while', but differ in nuance. うちに emphasizes doing something before a condition changes, e.g. 熱いうちに食べてね ('Eat it while it's hot.') 最中に highlights something occurring in the very middle of another action, e.g. 会議の最中に電話が鳴った ('The phone rang in the middle of the meeting.')

最中に and 途中に・途中で both mean 'in the middle of', but differ in nuance. 最中に emphasizes being right in the middle of something, e.g. 晩ご飯を作っている最中に電話が鳴った ('The phone rang when I was right in the middle of making dinner.') 途中に・途中で can overlap but also means 'on the way', e.g. 学校に行く途中で友達に会った ('I ran into a friend on the way to school.')

とき expresses the general time of an action or event, like 'when' in English. It can be used with nouns as in 雨の時 'when it rains', verbs as in 父が帰ってきた時 'when Dad came home', or adjectives as in 暑い時 'when it's hot', and can describe one-time or repeated actions. In contrast, 最中に describes something happening right in the middle of another action/event, e.g. 食事の最中に電話がなった ('My phone rang while I was in the middle of eating.').

Both express a continued state or action, but are used differently. 中 attaches only to nouns and means 'in the middle of (A)', e.g. 会議中 ('in the middle of a meeting'), 使用中 ('in use'). 最中に is used with verbs in the ている form or with nouns (using の) to mean 'right in the middle of (A), (B)', e.g. 食べている最中に話し掛けられた ('I was spoken to when I was right in the middle of eating.')

Both の間に and 最中に express 'while (A), (B)', but with slightly different nuance. 最中に is more emphatic, implying that (B) happened right in the middle of (A). In contrast, の間に simply states that (B) occurred during the general span of (A), and sounds more objective.

Both describe an interruption during an action, but with different focus. ～ところに・～ところへ highlights timing or the moment of interruption, e.g. 出かけようとしたところに電話がかかってきた ('Just as I was about to leave, the phone rang.') 最中に stresses that the action was in full progress, e.g. 晩ご飯を作っている最中に電話が鳴った ('The phone rang when I was right in the middle of making dinner.')

ているところだ conveys that an action is in progress at that moment, with the nuance that the speaker is busy with it, e.g. 晩ご飯を食べているところだ ('I'm in the middle of eating dinner.'). In contrast, 最中に is used to express that something else happened during that action, e.g. 晩ご飯を食べている最中に電話がかかってきた ('The phone rang while I was right in the middle of eating dinner.').