What kind of plant is this?
This popular plant for terrariums is Pilea depressa 'Baby Tears'. You can clip it a few leaves above the base, inside the terrarium and side-growth will fill in. Or, you could transplant it and keep as a general houseplant. Or both: root cuttings from the top part and leave the current plant in the container to grow side-shoots. Eventually it will get too large, as many terrarium plants do.
Here is a good article on general information and includes a list of good plants for terrariums if you get ready to re-plant yours. "Terrariums" G6520 from University of Missouri Extension Service: http://extension.missouri.edu/p/G6520
Thanks so much for your help! I'm new to gardening of any kind and never could grow anything until I retired recently. Now I'm growing all sorts of plants and getting excited. So afraid to transplant, though, that I might kill them. I'll check out the link you sent.
I hope you enjoy the plants, and don't fear death. Even the most experienced gardener has plants die. Sometimes because of a mistake, yet often just because nature isn't 100% predictable and plants don't always "read the rules".