If the meaning has changed according to neuroscience, what has it changed to?
I didn't mean the meaning has changed (first of all, because it is a new development that largely emerged thanks to the intellectually sterile "new atheism" has the attempt to define atheism in terms of a default epistemic position despite the inherent contradiction of such an attempt). It was a long time after the term existed before it could even refer to beliefs in deities rather than the relationship between a person and the gods, and in English it clearly meant disbelief in god:
"1. One who denies or disbelieves the existence of a God.
[?1555 Coverdale tr. Hope of Faythful Pref. f. iiiv, Eate we and drink we lustely, tomorow we shal dy. which al ye Epicures protest openly, & the Italian atheoi.]
1571 A. Golding in tr. J. Calvin Psalmes of Dauid with Comm. Ep. Ded. sig. *.iii, The Atheistes which say..there is no God.
1604 S. Rowlands Looke to It 23 Thou damned Athist..That doest deny his power which did create thee.
1699 Ld. Shaftesbury Inq. conc. Virtue i. i. 8 To believe nothing of a designing Principle or Mind, nor any cause or measure or rule of things, but Chance..is to be a perfect Atheist.
1876 W. E. Gladstone in Contemp. Rev. June 22 By the Atheist I understand the man who not only holds off, like the sceptic, from the affirmative, but who drives himself, or is driven, to the negative assertion in regard to the whole Unseen, or to the existence of God.
2. One who practically denies the existence of a God by disregard of moral obligation to Him; a godless man.
1577 M. Hanmer tr. Bp. Eusebius in Aunc. Eccl. Hist. iv. xiii. 63 The opinion which they conceaue of you, to be Atheists, or godlesse men.
1656 T. Stanley Hist. Philos. II. viii. 93 An Atheist is taken two waies, for him who is an Enemy to the Gods, and for him who believeth there are no Gods.
1667 Milton Paradise Lost i. 495 When the Priest Turns Atheist, as did Ely's Sons.
1827 J. C. Hare & A. W. Hare Guesses at Truth I. 65 Practically every man is an Atheist, who lives without God in the world."
(OED)
or for "atheism" the OED has
"Disbelief in, or denial of, the existence of a God. Also, Disregard of duty to God, godlessness (practical atheism).
1587 Sir P. Sidney & A. Golding tr. P. de Mornay Trewnesse Christian Relig. xx. 355 Athisme, that is to say, vtter Godlesnes.
1605 Bacon Of Aduancem. Learning i. sig. B3v, A little or superficiall knowledge of Philosophie may encline the minde of Man to Atheisme.
1711 J. Addison Spectator No. 119. ¶5 Hypocrisy in one Age is generally succeeded by Atheism in another.
1859 C. Kingsley Lett. (1878) II. 75 Whatever doubt or doctrinal Atheism you and your friends may have, don't fall into moral Atheism."
Webster defines it as:
a :a disbelief in the existence of deity
b :the doctrine that there is no deity
Neither of which is a "lack of" anything. More importantly, I raised the issue of neuroscience not because we use fMRI studies or something to determine what words mean, but that we can determine using neuroimaging the difference between lacking a belief (regardless of what one lacks a belief in) and the epistemic position atheists have merely by being able to identify themselves as such or use words like "god".