In an imperceptible manner or to an imperceptible degree:
— The power of the Secretary of State in London increased gradually but imperceptibly.

synonym: unnoticeably.

Roget 32: to a small extent [in a small degree], on a small scale; a little bit, a wee bit; slightly etc. adj.; imperceptibly; miserably, wretchedly; insufficiently etc. 640; imperfectly; faintly etc. 160; passably, pretty well, well enough.    [in a certain or limited degree] partially, in part; in a certain degree, to a certain degree; to a certain extent; comparatively; some, rather in some degree, in some measure; something, somewhat; simply, only, purely, merely; at least, at the least, at most, at the most; ever so little, as little as may be, tant soit peu [Fr.], in ever so small a degree; thus far, pro tanto [It], within bounds, in a manner, after a fashion, so to speak.    almost, nearly, well-nigh, short of, not quite, all but; near upon, close upon; peu s'en faut [Fr.], near the mark; within an ace of, within an inch of; on the brink of; scarcely, hardly, barely, only just, no more than.    about [in an uncertain degree], thereabouts, somewhere about, nearly, say; be the same, be little more or less.    no ways [in no degree], no way, no wise; not at all, not in the least, not a bit, not a bit of it, not a whit, not a jot, not a shadow; in no wise, in no respect; by no means, by no manner of means; on no account, at no hand.    ... show more