Simple Definition:

A consonant is a type of speech sound [a sound made by speaking] that is made by blocking or restricting the air flow through the mouth with the lips, tongue, or teeth.

Examples: b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y, and z are all consonants.

Very Simple Definition:

A consonant is a sound you make with your mouth by blocking the air. Examples are b, c, d, and s.