How to connect an iMac 2011 to an HDMI laptop?
Mactracker (backed up by Apple's specs here) indicates that the Thunderbolt port is also a video output:


Thunderbolt:    1 - Thunderbolt 1 (up to 10 Gbps)
Display:    Thunderbolt digital video output (Support for up to two Thunderbolt displays)


You should be able to use a MDP to HDMI adapter like this one from Belkin.

In our testing at my workplace, the Belkin adapters are the most robust.