im probably wrong but i think elm tree

Leaf shape doesn't look right for an elm to me but it is certainly a tree seedling so, unless you have a very large garden, would suggest you pull out now.

It's only a weed if you don't want it.

If you pull it, you might find remnants of the seed it came from. That might help with ID. Where in the US are you or do you know your grow zone?

It looks like an Aspen sucker, to me. Do you garden in the far northern States, or the Rocky Mountains or Sierra Nevada?

I live in Pennsylvania. On the east side.

Hmm..well, unless your neighbor has an Aspen--not impossible, but improbable there--it's got to be something else. Can we see a better shot of where the leaf joins the stem, especially near the growing point?