The Spanish word 'milagro' (meaning 'miracle') comes from the Latin word 'miraculum', meaning 'miracle' or 'wonder'. 'Miraculum' itself was formed by combining the Latin verb 'mirari' ('to wonder at') with the instrumental suffix '-culum'. This combination literally created a word meaning 'something that causes wonder'. The Latin 'miraculum' evolved into Old Spanish 'miraglo', which then developed into the modern Spanish 'milagro' through a common sound change where 'gl' became 'gr'.

The Spanish verb 'mirar' ('to look at, to watch') comes from the same Latin root 'mirari'. While 'milagro' retained the sense of something wonderous or miraculous, 'mirar' evolved to focus on the act of looking or watching. Another related Spanish word is 'admirable' ('admirable'), which also shares the root meaning of something that causes wonder or amazement.

Several English words are related to 'milagro' through the shared Latin ancestor 'mirari'. These include 'miracle', which is a direct cognate, as well as 'mirror' (originally something to wonder at), 'admire', and 'marvel' (through Old French). All these words retain some connection to the original concept of wondering or marveling at something extraordinary.