essentially, generally, in the main, largely, chiefly, primarily, in general, in the first place, principally.
Sarmiento's book was written, not only or mainly
to supply interesting information, but with an object.
Where the objective of the expedition is not the local naval base, the choice of a position for the covering squadron will turn mainly
on the amount of support which the army is likely to require.
It has been said with truth that the secret of a garden's beauty lies mainly
in its background.