What is the "‡" symbol meaning in a reaction mechanism? I was studying hydroboration from Clayden's Organic Chemistry [1, p. 1281] and the transition state had “‡” symbol in the upper right corner of the activated complex:


  We know that this is not the whole story because of the stereochemistry. Hydroboration is a syn addition across the alkene. As the addition of the empty p orbital to the less substituted end of the alkene gets under way, a hydrogen atom from the boron adds, with its pair of electrons, to the carbon
  atom, which is becoming positively charged. The two steps shown above are concerted, but formation of the C–B bond goes ahead of formation of the C–H bond so that boron and carbon are partially charged in the four-centred transition state.
  
  


What does this symbol mean? I have seen it before as a superscript at the Gibbs energy symbol.

References


Organic Chemistry; Clayden, J., Ed.; Oxford University Press: Oxford; New York, 2001. ISBN 978-0-19-850347-7.
The symbol is called "double dagger" (sometimes also "double cross") and is used to denote transition state (a maximum in an energy diagram; also often denoted with "*" or "TS") or a related physical property.
Note, however, that a transition state and an intermediate are two different terms.

The symbol has the peculiar origins: as written by H. Eyring, it's entirely the merit of the secretary of the department, Miss Lucy D’Arcy [1, p. 9]:


  The symbol, ‡, first appeared in the second paper dealing with the activated complex.
  In that manuscript a star was used to designate the activated state but 
  Miss Lucy D'Arcy, the departmental secretary, lacking a star used a plus and minus sign.
  Believing the typesetters would interpret this as a star it was left and by 
  their option the cross, ‡, has become the almost universally used sign for the activated state.


Also, note that according to IUPAC recommendations [2, p. 1080]:


  In accordance with previous IUPAC recommendations (IUPAC QUANTITIES (1988)) the symbol ‡ to indicate transition states ("double dagger") is used as a prefix to the appropriate quantities, e.g. $Δ^‡G$ rather than the more often used $ΔG^‡.$


In the future, I suggest to use services like Shapecatcher or Detexify to find the name of the symbol.
Finding its usage in chemistry afterwards is normally a trivial task.

References


Eyring, H. Models in Research. International Journal of Quantum Chemistry 1969, 3 (S3A), 5–15. DOI: 10.1002/qua.560030705.  
Muller, P. Glossary of Terms Used in Physical Organic Chemistry (IUPAC Recommendations 1994). Pure and Applied Chemistry 1994, 66 (5), 1077–1184. DOI: 10.1351/pac199466051077. (Free Access)