understatement
noun
[ ʌndəˈsteɪtm(ə)nt ]
• the presentation of something as being smaller or less good or important than it really is.
• "a master of English understatement"
Similar:
minimization,
trivialization,
euphemism,
understatedness,
restraint,
reserve,
underplaying,
underemphasis,
subtlety,
delicacy,
litotes,
meiosis,