A laminal consonant is a phone (speech sound) produced by obstructing the air passage with the blade of the tongue, the flat top front surface just behind the tip of the tongue in contact with upper lip, teeth, alveolar ridge, to possibly, as far back as the prepalatal arch, although in the last contact may involve as well parts behind the blade.  It contrasts with an apical consonant, produced by creating an obstruction with the tongue apex (tongue tip) only. Sometimes laminal is used exclusively for an articulation that involves only the blade of the tongue with the tip being lowered and apicolaminal for an articulation that involves both the blade of the tongue and the raised tongue tip.  The distinction applies only to coronal consonants, which use the front of the tongue.
Although most languages do not contrast laminal and apical sounds, the distinction is found in a number of languages:
Because laminal consonants use the flat of the tongue, they cover a broader area of contact than apical consonants. Laminal consonants in some languages have been recorded with a broad occlusion (closure) that covers all the front of the mouth from the hard palate to the teeth, which makes it difficult to compare the two. Alveolar laminals and apicals are two different articulations.
A very common laminal articulation is sometimes called denti-alveolar. It spans the alveolar ridge to the teeth but is a little farther forward than other alveolar laminal consonants, which cover more of the alveolar ridge and might be considered postalveolar. This occurs in French.
Part of the confusion in naming laminal consonants is quite literally a matter of point of view. When one looks at a person pronouncing a laminal alveolar or denti-alveolar, the tip of the tongue can be seen touching the back of the teeth or even protruding between the teeth, which gives them the common name of dental.
Acoustically, however, the important element is the place of the rearmost occlusion, which is the point that the resonant chamber in the mouth terminates. That determines the size, shape and acoustics of the oral cavity, which produces the harmonics of the vowels. Thus, French coronals are alveolar and differ from English alveolars primarily in being laminal rather than apical (in French, the tongue is flatter).
There are true laminal dentals in some languages with no alveolar contact, such as in Hindustani, which are different from French consonants. Nevertheless, the breadth of contact has some importance; it influences the shape of the tongue farther back and so the shape of the resonant cavity. Also, if the release of a denti-alveolar consonant is not abrupt, the tongue may peel off from the roof of the mouth from back to front and so shift from an alveolar to a dental pronunciation.
In the International Phonetic Alphabet, the diacritic for laminal consonants is U+033B ◌̻ COMBINING SQUARE BELOW (HTML