LATERAL DOMINANCE - Most trees generally  have a natural tendency to direct growth to the top branches (the APEX) which is called 'APICAL DOMINANCE'.  However the tendency in some trees is for growth to be directed laterally i.e. the growth tendency is outwards rather than upwards.  This is termed lateral dominance and tends to be exhibited by the shrubbier varieties , such as rhododenron (see Azalea (Rhododendron)).

The aim in bonsai is to decrease either of these dominant tendencies so that the subject looks more like a mature tree rather than a bush or a sapling.