Merriam-Webster Collegiate Dictionary - think

 I. verb  (thought; ~ing)  Etymology: Middle English thenken, from Old English thencan; akin to Old High German denken to ~, Latin tongere to know — more at thanks  Date: before 12th century  transitive verb  1. to form or have in the mind  2. to have as an intention thought to return early  3.  a. to have as an opinion ~ it's so  b. to regard as ; consider ~ the rule unfair  4.  a. to reflect on ; ponder ~ the matter over  b. to determine by reflecting ~ what to do next  5. to call to mind ; remember he never ~s to ask how we do  6. to devise by ~ing — usually used with up thought up a plan to escape  7. to have as an expectation ; anticipate we didn't ~ we'd have any trouble  8.  a. to center one's thoughts on talks and ~s business  b. to form a mental picture of  9. to subject to the processes of logical thought ~ things out  intransitive verb  1.  a. to exercise the powers of judgment, conception, or inference ; reason  b. to have in the mind or call to mind a thought  2.  a. to have the mind engaged in reflection ; meditate  b. to consider the suitability thought of her for president  3. to have a view or opinion ~s of himself as a poet  4. to have concern — usually used with of a man must ~ first of his family  5. to consider something likely ; suspect may happen sooner than you ~  • ~er noun Synonyms:  ~, conceive, imagine, fancy, realize, envisage, envision mean to form an idea of. ~ implies the entrance of an idea into one's mind with or without deliberate consideration or reflection I just thought of a good joke. conceive suggests the forming and bringing forth and usually developing of an idea, plan, or design conceived of a new marketing approach. imagine stresses a visualization imagine you're at the beach. fancy suggests an imagining often unrestrained by reality but spurred by desires fancied himself a super athlete. realize stresses a grasping of the significance of what is conceived or imagined realized the enormity of the task ahead. envisage and envision imply a conceiving or imagining that is especially clear or detailed envisaged a totally computerized operation envisioned a cure for the disease. Synonyms:  ~, cogitate, reflect, reason, speculate, deliberate mean to use one's powers of conception, judgment, or inference. ~ is general and may apply to any mental activity, but used alone often suggests attainment of clear ideas or conclusions teaches students how to ~. cogitate implies deep or intent ~ing cogitated on the mysteries of nature. reflect suggests unhurried consideration of something recalled to the mind reflecting on fifty years of married life. reason stresses consecutive logical ~ing able to reason brilliantly in debate. speculate implies reasoning about things theoretical or problematic speculated on the fate of the lost explorers. deliberate suggests slow or careful reasoning before forming an opinion or reaching a conclusion or decision the jury deliberated for five hours.  II. noun  Date: 1834 an act of ~ing has another ~ coming  III. adjective  Date: 1892 relating to, requiring, or stimulating ~ing