Stricture
From greatest to least stricture, speech sounds may be classified along a cline as stop consonants (with occlusion, or blocked airflow), fricative consonants (with partially blocked and therefore strongly turbulent airflow), approximants (with only slight turbulence), and vowels (with full unimpeded airflow). Affricates often behave as if they were intermediate between stops and fricatives, but phonetically they are sequences of stop plus fricative.
Historically, sounds may move along this cline toward less stricture in a process called lenition. The reverse process is fortition.
Read more about this topic: Manner Of Articulation