looks most similar to the catnip in my garden.

Resembles Chenopodium album - a commonly seen weed.

I also have many commonly seen weeds in my garden!

It's a Chenopodium species. Perhaps C album. A weed. A tip for identifying herbs is to use your nose. Both Lavender and Catnip (if that's what you meant. Or did you mean Catmint?) would be strongly scented.

Sometimes even common weeds such as this (aka goosefoot around here) can be attractive. But sadly they are still weeds. So focus on developing a better labeling marking system.

Dave

:( are you guys sure? I started this plant from seed but it was cropping up in pots I had labeled as pepper. Is it possible the soil hot contaminated with weed seed and they germinated?

also it has only now just been taken outside. previously was in a grow tent.

Definitely possible that weed seeds germinated in your pots. I see it all the time in my pots. And I too don't always get everything labeled. Every year I pledge to do better next season! Anyway, I have a pot that I labeled catnip and have been growing now 2 seasons. It looks a lot like what you have. So I guess I've been watering and feeding a weed! Explains why my cats weren't the least bit interested in it!

Well like floral said - use your nose. Catnip has a very distinctive smell. So do basil or mint or lavender. And unless you used sterile seed starting mix then weed seeds sprouts are common.

Dave

Leaves don't look like catnip. Catnip leaves are more strongly veined, with more rounded serrations. I agree about Chenopodium. As to weeds in a starting mix, it depends where the starting mix came from. Was it sterilized? Or did it include unsterilized soil from outside. If the latter, you'll get plenty of weeds.

'Are you sure?'

Yes 100%. Not only that it's not any of your herbs but also that its a Chenopodium.

What was the source of your seeds? Not only can seed come in with potting mixes, you can also fall foul of unscrupulous sellers.

gotcha. this is my first year starting from seed. I ordered online from pepper Joe's.... some from wal Mart.
the potting mix was outside in my shed for some time and it's possible I reused some potting soil from something I kept outside... thanks guys!

Most members of the mint family have square stems.

Definitely looks like a weed that grows in my yard and my flower pots.