I HAVE iMac (21.5-inch, Mid 2011) CAN I GET AN ADAPTER USB TO HDMI TO WORK FOR A SECOND DISPLAY?

What external display do you want to connect to the 2011 iMac?

most displays have more that one type of input. (VGA, DVI, HDMI, Display Port)

Basically you can use any of the following adapters with the 2011 iMac.

a Mini DisplayPort to VGA adapter

a Mini DisplayPort to DVI adapter

a Mini DisplayPort to HDMI adapter

a Mini DisplayPort to Display Port cable

Not USB to HDMI but you can get a Minidiplayport to HDMI adapter from Apple. You obviously will need a minidisplay port cable too, this cable will fit in your Thunderbolt port.