A thought form is the etheric picture, pattern or form created on the mental plane by the active mind -somewhat like an etching- that dissolves easily unless energized by the emotional nature. Real thought is the life principle of any thought-form. When this real thought or vital principle is withdrawn, the thought-form exists for a while as a corpse in the mental sea until it disintegrates. All physical forms originally were thought-forms.
Definition from The New Dictionary of Spiritual Thought by Carol E. Parrish-Harra, Ph.D.
The figures below are paintings of thought-forms clairvoyantly observed by Charles Leadbeater from the book Thought-forms.
12. A Thought of Love Peace and Protection for a Loved One
13. Selfish Grasping Affection (The I've got my hooks in you thought form)
15.INVOCATION 17. EVOCATION
INVOCATION. An invocation is a petition for support from a higher power; to call forth through prayer, incantation, concentration, projection or visualization, e.g., Let love prevail. It is a method of summoning power from an unseen force, source, or level to influence the physical world in a specific way. Figure 15. is a thought form of the "upward rush of devotion". Leadbeater describes this (fig. 15) as a splendid spire of highly developed devotion. He describes it as the outrush into manifestation of a grand emotion rooted deep in the knowledge of fact. The man who makes such a clearly defined thought-form as this is one who knows in whom he has believed. The determination of the upward rush of energy points to courage as well as conviction. Figure 15 clearly demonstrates the projection of energy upward in the form of a thought.
EVOCATION. Evocation is the coming forth of energy or spirit in response to an invocation (calling forth with intention). Figure 17. shows the response to the invocation shown in fig. 15 in the form of a downpour of light. Leadbeater states that on every plane of our solar system, the Solar Logos pours forth His light, His power, His Life, and naturally it is on the higher planes that this out-pouring of divine strength can be given most fully. The descent from each plane to that of the next below it menas an almost paralysing limitation. Yet, there are conditions under which the grace and strength peculiar to a higher plane may be in a measure brought down to a lower one and may spread abroad with wonderful effect. This seems to be possible only when a special channel is for the moment opened; and that work must be done from below by your effort. Whenever your thought or feeling is selfish (fig. 13), the energy which it produces moves in a closed curve, and thus inevitably returns and expends itself upon it own level; but when the thought or feeling is absolutely unselfish, its energy rushes forth in an open curve, and thus does not return in the ordinary sense, but pierces through into the plane above because only in that higher condition, can it find room for expansion. But in thus breaking through such a thought or feeling holds open a door ( to speak symbolically) of dimension equivalent to its own diameter, and thus forms the requisite channel through which divine force appropriate to the higher planes can pour itself into the lower with marvelous results, not only for the thinker but for others. Figure 17 symbolizes this response to invocation, and to indicate the great truth that an infinite flood of the higher type of force is always ready and waiting to pour through when a channel is offered, just as water may be said to be waiting to pour through the first pipe that may be opened (p. 38).
The result of the descent of divine life is a very great strengthening and uplifting of the maker of the channel, and the spreading all about him of a most powerful and beneficent influence. This effect has often been called an answer to prayer, and has been attributed to "special interposition of Providence", instead of the unerring action of the great and immutable divine law.
The energy exchanged described above parallels the process that occurs when the Soul Invocation is sounded. The projection of energy from below creates the channel which is filled with light and spiritual energies from above.
The science of invocation and evocation will be the primary method of communicating with Deity and higher forces during the Aquarian Age.
Power of Thought Quotes on thought-forms by the Tibetan Master
Online book on Thought-Forms by Annie Besant and C.W. Leadbeater