Aspiration

Aspiration is a feature in languages where saying a consonant gives out a puff of air. For example, if you dangle a piece of paper in front of your mouth, you will see it move if you say an aspirated, or breathy, consonant. If the paper does not move, then it is unaspirated, or not breathy. In English, voiceless stops and fricatives that happen at the beginning of a word are aspirated, which are the sounds 'p', 't', 'k', and 'ch' (which are written as /p/, /t/, /k/, /t͡ʃ/ in IPA in the same order). In IPA, aspirated sounds can be written with an ʰ symbol afterwards, as in /pʰ/, /tʰ/, /kʰ/, and /t͡ʃʰ/. For example, the first sound in the words "pick", "tick", "kick", and "chick" are all aspirated. In IPA, they are written as /pʰɪk/, /tʰɪk/, /kʰɪk/, and /t͡ʃʰɪk/ in the same order. However, voiceless stops and fricatives that appear after the first sound are unaspirated. For example, while the /p/ in "pit" is aspirated, the /p/ in "spit" and the /p/ in "tip" are not, so they would not be marked with an ʰ symbol afterwards. Below are more examples of aspiration.
English has no aspirated voiced sounds, but Hindi does. They are normally written with an 'h' after the consonant letter. An example of this is the name Bhattacharya, in which 'bh' is an aspirated 'b' sound, so it would be written as /bʱ/ in IPA.
In Mandarin Chinese, there are no voiced stops, fricatives, or affricates, so the only way to tell them apart is by aspiration. In Pinyin, aspirated sounds are written like voiceless sounds in English, so the sounds /pʰ/, /tʰ/, and /kʰ/ are written as 'p', 't', and 'k' in the same order, but unaspirated sounds are written like voiced sounds in English, so the sounds /p/, /t/, and /k/ are written as 'b', 'd', and 'g' respectively. For example, the word "Gaokao" would be written in IPA as /kau̯.kʰau̯/. In Wade-Giles spelling, aspirated stops, fricatives, or affricates are written with an apostrophe after the letter instead of using a different letter like in Pinyin, so "Gaokao" would be written as "Kaok'ao" in Wade-Giles.