[Solved]-What is JavaSE-18 in eclipse vs. my installed jdk1.8.0_73-eclipse

if you know there to find the "installed jres" in the eclipse preferences, just expand the "installed jres" node and select "execution environments", and things should become clear.

see https://wiki.eclipse.org/execution_environments for details of what they are.

execution environments (ees) are symbolic representations of jres. for example, rather than talking about a specific jre, with a specific name at a specific location on your disk, you can talk about the j2se-1.4 execution environment. the system can then be configured to use a specific jre to implement that execution environment.

obviously that text was written some time ago, but your javase-1.8 execution environment should automatically map to a java 8 installed jre. if you have more than one java 8 installed jre, you can choose which you prefer.