Junior — anyone who started after you.
If someone is your senpai, you are their kouhai. It is less used as direct address than senpai — you rarely call someone 後輩 to their face — and more a way of describing a relationship: 後輩に教える, teaching your juniors.
The mirror of 先輩: 後 (after) replacing 先 (before).
Describes, rather than addresses. Calling someone 後輩 directly can sound like you are putting them in their place.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to Japanese — it uses 後輩 the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook Japanese.
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