A bonus. See LOA 37 (below is a portion of the LOA)

-----------
Retention Pay

1. Retention Amount

The following are the minimum retention amounts, which may be increased at the discretion of the Company, provided that sum of the First Officer retention and his hourly rate annualized must not exceed the sum of the Captain retention and the Captain hourly rate annualized at his longevity level:

a. Captains

Pilots who are being paid in accordance with the Captain pay scale on the date of payment will receive the following minimum retention payments, provided he meets the conditions of paragraph C.2., below:

May 15, 2017 $2,000
Oct 15, 2017 $4,000

b. First Officers

Pilots who are being paid in accordance with the First Officer pay scale on the date of payment will receive the following minimum retention payments, provided he meets the conditions of paragraph C.2., below:

May 15, 2017 $3,000
Oct 15, 2017 $5,000
---------
Those numbers stated above are minimum amounts. The company recently said that the FO retention is going to go up to 10k per each payment date, totaling 20,000 dollars.