Lacuna is a Latin word which has been taken into English - but only for academic writing, in most cases. (It means 'gap', and is mostly used to refer to particular sorts of gaps: missing sections of an historical record, etc. It is pronounced with an '-oo-' vowel in the second syllable, 'lack-OO-ner', IPA: /læ 'kuː nə/.) As it is a Latin word being used in academic work, always write the plural in the Latin way: lacunae. (If you want to show off your word-processing skills, you can risk using the æ ligature digraph to write lacunæ; but that may seem affected.) Pronounce it 'lack-OON-ee', IPA: /læ 'kuː niː/.