Feynman diagram for the 2-point one-loop 1PI diagram

Let's break down each term:

• $phi^3$ means that each interaction vertex has three lines attached to it
• 2-point means the diagram has two external lines
• 1-loop means that any other lines need to be contracted to form a single loop

Now for these conditions, as you can verify, there are two possible diagrams [source: physicspages.com, Lancaster]:

But the final condition, being 1PI, means that it should not be possible to cut an internal line such that the diagram separates into two parts. That eliminates diagram (b), leaving diagram (a) as the 1PI, 2-point, 1-loop diagram for $phi^3$ theory.