When you are not sure what's the exact name of package like icedtea-7-plugin or icedtea-8-plugin, etc. Do a package name wise search before installing, for example search for "icedtea" word as follows:
$ sudo apt-get update
$ apt-cache search icedtea
This will display the matching package with word "icedtea" as follows:
icedtea-8-plugin - web browser plugin based on OpenJDK and IcedTea to execute Java applets
icedtea-netx - NetX - implementation of the Java Network Launching Protocol (JNLP)
icedtea-netx-common - NetX - implementation of the Java Network Launching Protocol (JNLP)
icedtea-plugin - web browser plugin to execute Java applets (dependency package)
By this you can decide which one to install..!
With the above output, use:
$ sudo apt-get install icedtea-8-plugin
The prior searching of package name will be helpful for any packages you need to install. If you are having back port issue, for the case of Ubuntu 14.04, then you may refer answer from @jthetzel.