Someone who attends or participates in a congress or large formal assembly.
From congress + -er (one who). This is a productive English suffix creating agent nouns, though 'congresser' never became standard compared to 'congressman' or 'congresswoman.'
English speakers preferred 'congressman' over 'congresser,' showing how we pick and choose which agent nouns stick—similar to how 'firefighter' won over 'fireman' in modern usage.
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