A nasalised sound is one which has simultaneous airflow through the oral and nasal cavities. Nasalisation may be contrastive, as in French which has the nasalised vowels ɛ̃ ɑ̃ ɔ̃ œ̃ The presence or absence of nasalisation may distinguish words, as in fait fɛ (=done) vs fin fɛ̃ (=end). More generally, languages have nasalised segments at the phonetic level. Vowels and approximant consonants are regularly nasalised in the vicinity of nasal stops. Compare the lateral segments in English fill fɪɫ and film fɪɫ̃m.
As can be seen from the above, nasalisation is symbolised with a tilde above (U+0303) the relevant symbol.