A consonant cluster (sometimes known as a consonant blend) is a group of consonants that appear together in a word without any vowels between them. When reading clusters, each letter within the cluster is pronounced individually.

Sometimes in certain consonant clusters (a string of two or more consonants in a word) the sounds may be reduced or dropped. Below are some of these reductions.

Listen to the pronunciation of these examples. The first word is with proper enunciation, the second with the reduced sounds. Then listen to the words  in sentences.

Because most dictionaries do not transcribe individual allophones, the first transcription shown below is the more common dictionary transcription. The second transcription can help non-native English speakers better understand the common pronunciation used by native English speakers.
1. /tr/    →    /ʧr/      tree: /tri/            → /ʧri/
2. /dr/   →    /ʤr/     dream: /drim/   → /ʤrim/
3. /str/  →    /ʧr/       street: /strim/   → /sʧrit/

trace          track            tract           tractor           trade            tradition
traffic        tragedy        tragic         trail               trailer           train
trainer       traipse          trait           traitor            tram             tramp
trample     trampoline   trance        tranquil         transfer         translate
transmit    transplant     transport   trap                trapeze         trash
trauma      travel

Many of these consonant blends probably look familiar to you since we all use words with them on a daily basis.

The consonant clusters belonging to a single syllable are known as intra-syllabic clusters whereas the consonant clusters belonging to two different syllables in a single word are known as inter-syllabic clusters. Thus, in linguistics, a consonant cluster also known as consonant blend is a group of consonants which have no intervening vowels in between them. The maximum possibility of consonant cluster is three consonants in the beginning and four in final position.