Main Entry: hy·dro·col·loid Pronunciation: -'käl-"oid Function: noun : any of several substances that yield gels with water (asalginic acid salts, agar, carrageenan, and related polysaccharide gums) and that are used especially as protective colloids and as impression materials in dentistry —hy·dro·col·loi·dal/-k&-'loid-&l, -kä-/adjective