Vehicle airbag

1. An assembly, comprising:

a seatback;

a side airbag supported by the seatback and inflatable to an inflated position including a top panel having a pair of opposing side edges and a front edge extending between the side edges;

the side airbag in the inflated position including a bottom panel;

the side airbag including a main panel fixed to the top panel at a seam extending along the side edges and the front edge, the main panel extending from the top panel to the bottom panel in the inflated position; and

a tether fixed to the bottom panel and the seatback, the tether connected to the bottom panel at the seatback and extending away from the seatback along the bottom panel.

2. The assembly of claim 1 , wherein the bottom panel includes a pair of opposing side edges and a front edge extending between the side edges, the main panel fixed to the bottom panel at a second seam extending along the side edges and the front edge of the bottom panel when the side airbag is in the inflated position.

3. The assembly of claim 2 , wherein a distance between the seam and the second seam at the seatback is less than a distance between the seam and the second seam at the front edge of the bottom panel.

4. The assembly of claim 1 , wherein the tether is triangular.

5. The assembly of claim 1 , further comprising a second tether fixed to the top panel and the seatback.

6. The assembly of claim 1 , wherein the main panel is a single sheet of material.

7. The assembly of claim 1 , wherein the side airbag is inflatable in a seat-forward direction from an uninflated position to the inflated position.

8. The assembly of claim 1 , wherein the seam at one of the side edges of the top panel is spaced from the seam at the other of the side edges along a cross-seat axis.

9. The assembly of claim 1 , wherein the opposing side edges of the top panel are spaced from each other by at least 50 millimeters in the inflated position.

10. The assembly of claim 1 , wherein the top panel is domed upwardly from the seam.

11. The assembly of claim 1 , wherein the seam is U-shaped.

12. The assembly of claim 1 , wherein the seam extends from a first end to a second end, the first end and the second end at a same position relative to a seatback-vertical axis.

13. An assembly, comprising:

a seatback having a vehicle-inboard side;

an airbag supported by the seatback at the vehicle-inboard side and inflatable to an inflated position including a top panel having a pair of opposing side edges and a front edge extending between the side edges;

the airbag in the inflated position including a bottom panel;

the airbag including a main panel fixed to the top panel at a seam extending along the side edges and the front edge, the main panel extending from the top panel to the bottom panel in the inflated position; and

a tether fixed to the bottom panel and the seatback, the tether connected to the bottom panel at the seatback and extending away from the seatback along the bottom panel.

14. The assembly of claim 13 , further comprising a second seatback spaced from the seatback along a cross-vehicle axis by a distance, and the side edges of the top panel are spaced from each other along the cross-vehicle axis by at least half the distance between the seatback and the second seatback.

15. The assembly of claim 13 , further comprising a center console, wherein the airbag in the inflated position is directly above the center console.

16. The assembly of claim 13 , further comprising a seat bottom supporting the seatback, the airbag in the inflated position extending along the seat bottom.

17. The assembly of claim 13 , wherein the seam extends along a cross-vehicle axis and a vehicle-longitudinal axis.

18. The assembly of claim 13 , wherein the bottom panel is spaced from the top panel along a vehicle-vertical axis, and the main panel is fixed to the bottom panel at a second seam extending about the bottom panel.