From Operating System point of view, port can have only two states either Open or Close. Irrespective of port state from OS views, NMAP detects six type of port states and these states are listed as below
Open =>This state is same as Operating System open state. If any port is Open then it will accept incoming connection.
Close => This state of port also has same meaning in Operating System and NMAP. Port is close state can’t accept incoming traffic.
Filtered => This NMAP state says that port is query of port status has been blocked by packet filtering firewall.
Unfiltered => This NMAP state says that port is accessible but NMAP unable to conclude port is Open or Close.
Open|Filtered => There are a few NMAP scan types where Open ports don’t respond. Open|Filtered state means NMAP unable to determine if the port is open or filtered.
Close|Filtered => Unable to determine Port is Close or Filtered by firewall.
We can notice in above screenshot that, in normal SYN scan port 22 is showing in Open state while same port 22 appearing as Open|Filtered in XMas scan.