Per the IRS Instructions for Schedule A, page A-11:

Line 21 
Unreimbursed Employee Expenses
Enter the total ordinary and necessary job expenses you paid for which you weren’t reimbursed. (Amounts your employer included in Box 1 of your Form W-2 aren’t considered reimbursements.)

An ordinary expense is one that is common and accepted in your field of trade, business, or profession. A necessary expense is one that is helpful and appropriate for your business. An expense doesn’t have to be required to be considered necessary.