Clip or constrain TIN to inside polygon?

[TimHayes]

I am using ArcGIS 10.2.1.

I have elevation contour lines.

I have clipped these contours to one area where I want to create a TIN.

I have outlined the area I want to generate a TIN with a polygon.

How do I constrain the TIN creation to only inside this polygon?

[BondHarper]

When using the Create TIN from Features tool, define your clip area polygon as a "hard clip". If you've already generated a TIN, you can Add Features to TIN (also hard clip).

[JeffreySwain]

I would only use the 'Hard Clip', if you have made your constraining polygon 3D.  If you have a normal boundary, then use the soft clip so that the TIN will constrain, but the values on the edge of the TIN will not take that of the constraining boundary. 

Take a look at this help documentation for more information. I would also urge you to consider terrains to model your area.