The first ionisation energy of an element or molecule is the energy required to move the most loosely held electron from one mole of gaseous atoms of the element or molecules to form one mole of gaseous ions with electric charge +1.
ðə fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒi əvən ˈɛlɪmənt ɔːɹ mˈɑːlɪkjˌuːl ɪz ðɪ ˈɛnɚdʒi ɹᵻkwˈaɪɚd tə mˈuːv ðə mˈoʊst lˈuːsli hˈɛld ᵻlˈɛktɹɑːn fɹʌm wˈʌn mˈoʊl ʌv ɡˈæʃəs ˈæɾəmz ʌvðɪ ˈɛlɪmənt ɔːɹ mˈɑːlɪkjˌuːlz tə fˈɔːɹm wˈʌn mˈoʊl ʌv ɡˈæʃəs ˈaɪɑːnz wɪð ᵻlˈɛktɹɪk tʃˈɑːɹdʒ plˈʌs wˈʌn.