Approximants are speech sounds that involve the articulators approaching each other but not narrowly enough nor with enough articulatory precision to create turbulent airflow. Therefore, approximants fall between fricatives, which do produce a turbulent airstream, and vowels, which produce no turbulence. This class of sounds includes lateral approximants like (as in "less"), non-lateral approximants like (as in "rest"), and semivowels like and (as in "yes" and "west", respectively). 

Before Peter Ladefoged coined the term "approximant" in the 1960s, the term "frictionless continuant" referred to non-lateral approximants. 

Some approximants resemble vowels in acoustic and articulatory properties and the terms "semivowel" and "glide" are often used for these non-syllabic vowel-like segments. The correlation between semivowels and vowels is strong enough that cross-language differences between semivowels correspond with the differences between their related vowels. 

Vowels and their corresponding semivowels alternate in many languages depending on the phonological environment, or for grammatical reasons, as is the case with Indo-European ablaut. Similarly, languages often avoid configurations where a semivowel precedes its corresponding vowel. A number of phoneticians distinguish between semivowels and approximants by their location in a syllable. Although he uses the terms interchangeably, remarks that, for example, the final glides of English "par" and "buy" differ from French "par" ('through') and "baille" ('tub') in that, in the latter pair, the approximants appear in the syllable coda, whereas, in the former, they appear in the syllable nucleus. This means that opaque (if not minimal) contrasts can occur in languages like Italian (with the i-like sound of "piede" 'foot', appearing in the nucleus: , and that of "piano" 'slow', appearing in the syllable onset: ) and Spanish (with a near minimal pair being "abyecto" 'abject' and "abierto" 'opened').
Who came up with the term approximant?
When?
What word was used before that?
What did that represent?
Are they fricatives?
What speech sound makes no turbulence?
What do languages frequently avoid?
What does location within a syllable help distinguish?
Can Italian have opaque contrasts?
What about Spanish?
Peter Ladefoged
Before Peter Ladefoged coined the term "approximant" 
in the 1960s,
Before Peter Ladefoged coined the term "approximant" in the 1960s,
frictionless continuant
the term "frictionless continuant" referred to non-lateral approximants. 
speech sounds that involve the articulators approaching each other but not narrowly enough nor with enough articulatory precision to create turbulent airflow
Approximants are speech sounds that involve the articulators approaching each other but not narrowly enough nor with enough articulatory precision to create turbulent airflow
no
Therefore, approximants fall between fricatives, which do produce a turbulent airstream, and vowels, which produce no turbulence. 
vowels
and vowels, which produce no turbulence. 
they avoid configurations where a semivowel precedes its corresponding vowel
languages often avoid configurations where a semivowel precedes its corresponding vowel.
semivowels and approximants
A number of phoneticians distinguish between semivowels and approximants by their location in a syllable.
yes
This means that opaque (if not minimal) contrasts can occur in languages like Italian 
yes
This means that opaque (if not minimal) contrasts can occur in languages like Italian (with the i-like sound of "piede" 'foot', appearing in the nucleus: , and that of "piano" 'slow', appearing in the syllable onset: ) and Spanish (with a near minimal pair being "abyecto" 'abject' and "abierto" 'opened').