'Loísmo' refers to two different morpho-syntactical constructions in Spanish. The first is the opposite of leísmo, that is, while leísmo uses 'le' as a masculine direct object pronoun as well as indirect, in loísmo the pronoun 'lo' is used for both the indirect and direct objects. However, where 'le' would change to 'se', like in the series 'Da + le(lo) + lo 'give him it (Give it to him)', the 'lo' used to represnt the indirect object still changes to 'se', to you get 'Dáselo'
'Loísmo' also refers to the substitution of 'lo' for the nominative pronoun 'él', so that 'Lo me gusta' means 'I like him' and 'lo come' is 'he eats'. I believe this is fairly rare though.
A person with loísmo is called 'loísta'.