Is it illegal to enter an abandoned house?

There is no simple answer to this question.

Abandoned has different legal meaning in different jurisdictions.

For sure if you need to scale a fence to get in you're trespassing. If there's some variation of a "No Trespassing" sign on the premises, you're trespassing. If you need to break a padlock you're trespassing, and breaking and entering.

But, even is there are none of these deterrents the law may find it trespassing. In some jurisdictions you will be forgiven even if it's trespassing if you didn't know it was. In others not. So the entire thing is muddy.