What is thoughtcrime in 1984?
Thoughtcrime is thinking of anything that the Thought Police and the Party deem is illegal. Illegal is anything that creates individuality--not the best thing for the Party as a whole. They want Party members to remain isolated so that no one can band together and rebel like the Proles.
For example, Winston continually looks around during the Two Minute Hate and peruses the crowd for others who might give away for even the briefest of seconds that he/she might also be against Big Brother like he is. This is where he thinks O'Brien is against the whole Party thing, and also where he repeatedly finds himself staring at Julia before they formally meet.
Julia's passing the note to Winston is also thoughtcrime. It proves that she has been "thinking" for a while about having a relationship outside of the Party which is forbidden. Therefore, the affair she and Winston have is absolutely illegal.
It is the representation of any form of individualism.
Thought crime is the holding of any unorthodox ideas or thoughts
Thoughtcrime can be defined as anything one thought that was not aligned with what "the party" thought. It was a crime if anyone suspected that you were thinking anything other than what "Big Brother" wanted you to think or to know. An example from the story would be when Winston was in the cafeteria and the announcement referencing how well the economy was came on. This included things such as how much more food they had as well as how many pairs of shoes were made, etc., when in fact there was less. In the midst of the announcement, Winston was thinking just the opposite and thinking that if there was so much, why were others asking for items like razor blades from each other.