"Cereticiaun" is almost certainly "Ceredigion", a name now associated with modern Ceredigion county, with which the area may have been more or less coterminous. Traditionally the name is supposed to originate with the king of the area Ceredig (though not necessarily the Ceredig who died in 615 CE, see below). The development of the placename can be seen from its appearances in the Annales Cambriae (See Other Works) in the following references from Version A (attached to Manuscript A: c.1100 CE but could be down to 977) and B (1286+):

A+B[615]: Ceretic obíít. [Ceredig dies]
A+B[807]: Arthgen rex[A]/dominus[B] cereticiaun[A]/Keredigeaun[B] moritur / Eclipsis solis[B]. [Arthgen king/lord of Ceredigion dies. Solar Eclipse (Eclipse in version B)].
A+B[871]: Guoccaun[A]/Gugan[B] mersus est, rex cereticiaum. [Gwgon, king of Ceredigion, is buried/drowned].
A+B[894]: Anaraut cum anglis uenit uastare cereticiaun[A]/Ceredigaun[B] et strat tui[A]/Stratewi[B] [Anaraut has come with the Englishmen to lay waste to Ceredigion and Ystrad Tywi].
B[987]: Gothfrit filius Haraldi cum nigris gentilibus vastavit Mon, captis duobus millibus hominum: reliqias vero Maredut secum asportavit ad Keredigean et ad Demetiam. [Gothfrit son of Harald at the head of that black/unlucky tribe has laid waste to Mon (Anglesey), captured two thousand men: Maredut has taken away true relics to Ceredigion and to Demetia (~South Wales*); ~the rest of Dyfed]
B[992]: Guyn filius Eynaun, duce Edelisi Anglico, dextralium Brittonum adiumento, regiones Maredut, id est Demetiam et Keredigeaun, Guhir et Kedweli, devastat. Tertio Menevia vastata est. [Guyn son of Eynaun, leader of the Edelisi Englishmen, with the help of the righthand Britons, devastates the region of Maredut, that is Demetia and Ceredigion, Gower and Kedweli. A third of Menevia is laid waste.
B[1072]: De Mungumeri Hugo vastavit Keredigiaun. [From Mungumeri (Montgomery) Hugo has laid waste to Ceredigion]
B[1093]: Ruses filius Teudur rector dextralis partis a Francis Brechenauc occisus est, pot cuius obitum Cadugaun filius Blediut praedatus est Demetiam pridie Kalendarum Maii. Circiter Kalendas Iulii Franci p[r?]imitus Demetiam et Keredigean tenueraunt, et castella in eis locaverunt, et abinde totam terram Britonum occupaverunt. Mailcholum Scottorum rex occisus est. [Rhys son of Tewdwr leader of the right part by Brechenauc (Brecon) French is killed, [because of?/following?] whose death Cadwgan son of Bleddyn has[?] pillaged Demetia the day before the Kalends of May (Start of May). Around the Kalends of July the Primate of France held Demetia and Ceredigion, and strongholds into the same they have placed, and therehence the whole of the country of Britain they have occupied. Mailcholum, king of the Scots is killed.
B[1094]: Willelmus Anglorum rex Normanniam adiit, quo ibi morante, Britanni iugum Francorum respuerunt, Wenedociam, Cereticam et Demetiam ab iis et eorum castellis emundaverunt, exceptis duobus, is est in Pembroc et aliud in Ricors. [William the Englishman, king of Normandy attacks, who is staying there, the Britons have rejected the yoke of the French, from Wenedocia (~North Wales), Ceredigion and Demetia, and their strongholds they have cleansed, excepting two, these are in Pembroke and also in Rhyd-y-gors (Carmarthen Castle).
B[1095]: Franci devastaverunt Goher et Kedweli et Stratewi. Demetia Ceretica et Sratewi deserta manent. Mediante autumno rex Anglorum Willielmus contra Britones movit exercitu, quibus Deo tutatis, vacuus ad sua rediit. [The French have laid waste to Gower and Kedweli and Ystrad Tywi. Demetia, Ceredigion and Ystrad Tywi remain deserted. Half way through Autumn, the English king William has moved in exercises against the Britons, who are guarded by God, returning to his with nothing.