The sec() command returns the secant (the reciprocal of the cosine) of an angle measure. Naturally, the result depends on the angle mode the calculator is in: radian, degree, or (in AMS version 3.10) gradian. You can also use one of the r, °, G marks to specify an angle mode.
The sec() command, along with 11 other trig and hyperbolic functions, was added with AMS version 2.07. It can be easily replaced on earlier versions with 1/cos(x), which is what it simplifies to anyway.
For many common angles, sec() can compute an exact result. Other angles, the calculator will leave alone unless it's in approximate mode (or unless you make it approximate), and then it will give a decimal approximation. As long as the calculator is in radian mode, sec() can be used with complex numbers as well.
:sec(60°) 2 :sec(x) 1/cos(x) :sec(π/2) undef
If sec() is applied to a list, it will take the secant of every element in the list. However, it can't be applied to matrices the way cos() can (this is probably an oversight; all the trig and hyperbolic functions that were present in all AMS versions work with matrices, but the ones added in version 2.07 do not).
260 - Domain error happens when taking sec() of a complex number in degree or gradian mode.