Definition of "at will"

I gif the witt, I gif the strenght, / of all thou sees, of brede & lengthe; / thou shall be wonder wise. / Myrth and Ioy to haue at will, / All thi likyng to fulfill, / and dwell in paradise.I give thee consciousness, I give thee strength over all thou seest, over all its breadth and length thou shalt be wondrously wise. Mirth and joy [you shall] have at will to fulfil all thy liking and dwell in paradise.

15th c., “[The Creation]”, in Wakefield Mystery Plays; Re-edited in George England, Alfred W. Pollard, editors, The Towneley Plays (Early English Text Society Extra Series; LXXI), London: […] Oxford University Press, 1897, page 6, lines 174–179

[...] there is no special incentive to adopt a design on the latest French pattern, in which the gear ratio can be changed at will to permit higher tractive efforts on freight duties as an alternative to higher speeds on passenger duties.

1960 December, “The first hundred 25 kV a.c. electric locomotives for B.R.”, in Trains Illustrated, page 727

And yet, United always carried the greater threat. Their movement in attack constantly confounded Southampton's defenders, with Rooney, Van Persie and Januzaj swapping places and eluding markers at will.

2013 October 19, Ben Smith, BBC Sport