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1.
a daily allowance, a daily fee

An employee can claim $120 per diem for 2 weeks towards food expenses.
by Balaji D August 27, 2006
 
2.
Money that your employer grants you will traveling so you can drink alcohol liberally. Employer's province this entitlement to relieve stress derived from travel, bosses, co-workers, etc.
Going out of town sucks, by at least I can drink $40 of per diem to make up for it.
by rizzoyo June 28, 2014